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It is software which hands over full or partial control of your computer system to any user who have created the malware or malicious software or who have developed it. If it penetrates into the computer system then its creator can do anything which he wants and can get full control of the computer system. The damage that can be done to the system or resources can be in the form of changing the name of any file or getting full access or deleting something from the system. 
Most of the malwares requires users for their actions and initiation. It can be sent to any user in the form of email attachment and can spread from any malicious website. It can also activated by clicking on any popup during any software installation and it can easily penetrate into the computer operating system. 
There are different types of malware such as viruses, worms, Trojan horses, backdoors, spyware, rootkit and key stroke loggers. These all malwares are different in types and functionality and damage to the system.
Many experts usually define malwares into two categories such as Family and Variants.
Family describes original piece of malware whereas variant is any different version of malicious software which belongs to the same type or family.
It is a small, hidden program that affects other software's by spreading into computer systems memory and leaving its copy into that software. It contaminated the system and delivers the payload to the system while executing the computer's memory, deleting or formatting the computers storage devices, degrading computers overall performance and infecting other computer files and injecting malicious software's into them. It is always initiated or starts its execution whenever user clicks on something unusual or suspicious or while its parent program is in running condition. 
Viruses are the oldest form of the malware which are written accidently in academic computer labs and experimental teenagers in late 1970s. 
Worm is software which self-replaces itself in computer memory and storage devices and it doesn't need any parent software or host program and any user interaction. It generally delivers a load on computers memory and storage and exploits the computers operating system and it rapidly replicates itself at computer network which slows down the network speed resulting in bad or no communication. It also exploit the software's installed in the computer system and it fills up the storage devices by replicating itself. Viruses and worms are the only sources and software's which are self-executable and propagates themselves without any user interaction on the internet and in the computer systems. 
Trojan horse is a computer software that appears as a useful software but it performs the functionality to weak the computer security and enabling any person to attack on the computer system. A Trojan horse may look like useful and attractive software which user intentionally install and doesn't knows that what he have done. Trojan horse instantly start its working as user installs it and it behaves like key loggers and it can send any information about the computer data and users secret information to the person who developed it or at some remote location. It can also be used to disable the computer systems security. 
A backdoor is a malicious code that is used to enable the attackers to remotely attack on the computer system and run their commands and software's and for unauthorized access. With the backdoor attackers can change and run any commands on the computers system and can install any software which ultimately send users information to any remote location and passwords of the user. Most of the Trojan programs and bot programs have that functionality like backdoors. Bot program is a program that enables the attackers to get access of the system remotely and have access of series of information systems simultaneously. Backdoor intentionally included in the software's to get the user experience without any attention of the user which exploits the users privacy. 
It is a form of malicious software which is used to capture the data from the computers input(Like mouse, keyboard) , output(screens) devices and storage devices and sends that data to a remote location to the attacker without the permission and attention of the user. User always didn't know about it that some software is sending his important information to somebody else. Some companies used this software for collecting information about the user behavior that what kind of websites user visits but it malicious nature enables it to send user passwords, credit card numbers and bank account numbers whenever user enters it in the websites. 
A keystroke logger is a program which runs it the computer memory without the knowledge of the user and it keeps a record of any thing which is being typed by the user like username, passwords, credit card numbers and bank account numbers. It saves all this information in any file and finally sends that file to any person across the network. This is the functionality which is in the Trojans as well. 
Malware is Profitable:
Malware is a serious business today and it is used by the cyber criminals and malicious actors to get sensitive information of different users without their attention. Cyber criminals get their account numbers and credit card information by using malicious software's and combination of different malicious software's. It is not a fun and game now days and is widely being used by different criminals for unauthorized access of the systems. 
Who are looking to exploit our systems:
In this era of technological advancements, there are different technologies that are being made for the betterment of the human beings but there are many users who are using these technologies for their own benefits and using those technologies not for the purposes for which its inventors have made it. Unfortunately every device and network which is rapidly used by the users is infected by these malicious software's. Technologies that are widely used by the computer operators are at hit list by these criminals. There are four kinds of criminals. 
Most of the computer viruses are written by the computer students. These students are new to computer studies and have learnt some computer language and write some program in it for their self-esteem.These kinds of viruses don'tspread over the different networks and computers and died at their disks and storage devices. Some times their authors used to send them to the antivirus companies to make a solution for it. 
The second groups of virus writers are young people and students who haven't yet mastered any programming language and writes computer virus due to the inferiority complex. These newly virus writers produce some viruses with numerous mistakes but with the rising of internet and websites technology they got knowledge about this technique. Different websites are being made which educate the computer users and virus writers to write specific kind of viruses and fully train them to who the spread it in the computer systems. These websites often provide readymade text and code to write the viruses and users have just a little things to change from it. 
Older and experienced virus writers fall into this category, the third and most dangerous group, which creates some very experienced and top computer programming professionals. These users know everything about the normal and abnormal process of the computer systems and they hunt the weaknesses in the security systems and software's and operating systems. 
Fourth group of virus and malicious software writers are the researchers. These are the programmers who invent and discover new ways of penetrating into the systems and writing the viruses. These researchers always look for the weaknesses and discover some holes into the systems and write the methods to how to write the viruses for penetrating into the system through those holes or write only the ways to how to write the virus and malicious software.They often actively promote their methods and ideas through internet which enables medium level programmers and virus writers to write the viruses to damage or fail any targeted software or operating system. 
By the emergence of the paid internet technologies many companies increase the service charges for these computer services. Then the users who doesn't have money or don't want to pay start trying to steal the account names and passwords of some other person to use these services at some others personnel expense.
In 1997 special Trojans are developed to steal password and usernames of AOL. And in 1998 many different Trojan software are being written for the other services of the internet. All these Trojans and viruses are being written by those developers and programmers and professionals who couldn't bear the expanses of these services. When the service charges of these services gets lower all these attacks decreases accordingly. 
This is the most dangerous category who writes viruses for any specialized purposes. These professional virus writers are called hackers. They write different malicious software's and viruses in any specialized category of interests. They write Trojan programs to steal bank accounts information, credit card information and to transfer money from any other persons account to their account. 
These criminals write specialized Trojan programs which are used to steal electronic money from their E-Wallets accounts. They write specialized Trojan programs which copy the information of access codes to accounts and send it to their master. Then these people use this information illegallyfor their benefits. This criminal uses the same methods for stealing the information of bank accounts of different users. They usually create a same page as of bank account and place it accordingly with the website; whenever user enters his bank or account information into it they instantly send that information to their master. They use Trojan spies for these purposes. They use social engineering techniques to force the user to enter the bank account information. They display negative messages like something will go wrong (bank will close his account) if he doesn't write the information into it. Then users write the information into it and they will get the information and make transactions. 
Grey market business personnel's are the criminals who are doing illegal things but on the brick of law. The imposes the different advertisements and links on the websites which ultimately goes to some paid web resource and different software which shows popups for payment for the full control of any fake website. These persons also require hackers to write some programs for them. They uses different software's that installs themselves after just one click and always prompt the user for the paid registration whenever he connects with the internet.
There is a relatively new type of cybercrime which informs the user that there is a spyware in the system of the user and if he wants to detect it and remove it then he have to click at this link. User just clicks only for the one time and it installs itself and prompts user to pay for removing the spywares. User enters his information and clicks on the links just to remove the spyware from his system. 
Evolution of Malware:
Before the internet has become very common, viruses are being used and write in machine language for the floppy disks to damage and interrupt their boot sectors. These are the executable which runs in memory and start their working whenever disks start booting. In start viruses are being written for the Apple and Mac OS. Then IBM PC came in the market and dos operating system came in being so users started writing software's for these two operating systems. These executable are dependent on users, they copy the floppies to their friends and other persons use and it transfers the virus to them. 
The first worm was originated and made for the network server and is made for the multitasking operating system UNIX in 1988. The first well known worm was Internet worm which was made to attack on the Sun OS and VAX BSD systems. Unlike virus this software doesn't affectsoft wares but it checks the network servers and finds some security holes in it and exploits it form those security holes. It runs as a process in the systems memory. All the worms are written using the same pattern and technique. 
With the rise of Microsoft windows and different technologies in 1990s and Microsoft uses macro language for its software's and different soft wares uses this language for performing different kind of functionalities. So macro viruses are being written in the macro language and are not used to infect executable and software setups. These are used to infect the documents and templates. 
Today worms are most commonly written for the windows operating system and some of them are written for the Linux and UNIX operating system as well. All the worms are written using same technique as of first worm known as Internet Worm developed in 1988. They scan the network and find some hole from it and start infecting all the computers on the network. Worms can spread with an incredible and amazing speed. Thousands of computers can be infected in a few seconds with a very famous worm known as SQL Spammer.
Evolution of Anti-malware:
Because of the great spread of viruses the first antivirus software became very popular in the computer software industry. The first anti-virus and anti-malware software is being written by Bernt Fix in 1987.
Viruses are being spread through floppy drives. Then antivirus came into being and there is only one problem with it that it wouldn't be frequently updated. During this time this antivirus software has to check the boot sectors of the disks and also all the executable of the different software's as well. But as internet came into being and is widely used then viruses are being spread on multiple systems easily. 
By the time antivirus soft wares are become common companies start developing professional antivirus software's. They start using different virus detection and removing techniques like signature based detection and heuristics. Antivirus soft wares of are made by following these two techniques.