The UCL Student Handbook defines plagiarism as: "The presentation of another person's thoughts or words or artefacts or software as though they were your own".
"Any quotation from the published or unpublished works of other persons must, therefore, be clearly identified as such by being placed inside quotation marks, and students should identify their sources as accurately and fully as possible. A series of short quotations from several different sources, if not clearly identified as such, constitutes plagiarism just as much as does a single unacknowledged long quotation from a single source."
"Equally, if a student summarises [rather than quotes verbatim] another person's ideas, judgments, figures, diagrams, or software, a reference to that person in the text must be made and the work referred to must be included in the bibliography."
Plagiarism includes use of essays written by other students, even when somewhat modified.
Tutors expect students to consult the relevant professional literature first-hand and to undertake research themselves. Essays from other students are NOT be reliable sources of information.
In cases where students undertake research together, use the advice, "talk together; write alone".
Reference lists must include all work cited in an essay and must include only items examined personally by the student.
Self-plagiarism involves submitting the same material for assessment multiple times.
No work, in part or whole, may be credited to a degree programme more than once.
Self-plagiarism includes use of a previous module essay in another module, including a 3rdyear dissertation. Some exams permit answers on questions you have written essays on, others do not.
Why avoid Plagiarism ?
We will find it. We are good at spotting it, and we have VERY powerful software. We do mind.
We don't have choice as there are University regulations, but we, personally, do mind as well.
Better to hand in nothing, or an attempt at an essay, than to plagiarise.
A score of zero for a the relevant course is the minimum punishment.
It is obvious to us when an essay suddenly shifts style, sophistication, grammar, comprehension, etc. and then shifts back - as obvious as if a professional sports player joins in an amateur game.
There are technical ways of spotting cut and paste, obvious to the trained eye.
We know the literature better than you know your favourite novels.
If in doubt, we usually know the textbook authors and can ask them.
We have Turn It in, an extremely powerful plagiarism detection programme.
It detects: Plagiarism from books, websites, essays (other UCL essays, other University essays, your other essays)
How to avoid Plagiarism ?
When you quote or summarise, make a footnote and give the reference of where this comes from.
Every essay should have a bibliography full details of all the works you have consulted for your essay.
Discuss together, write apart .
Fine (and encouraged) to discuss essay topics with other students, but when you sit down to write, you are on your own ! Do these things, and there will be no problems.
If you writeâ€¦
The ancient Greek word cosmeo has given us several words in modern English, such as cosmology (the study of the universe), cosmogony (the study of the origins of the cosmos) and cosmetic. The last may seem somewhat surprising, but cosmeo meant not only to order or arrange, but also had a sense of good order and beautiful, aesthetically pleasing order.
That is plagiarism. If you write
"The ancient Greek word cosmeo has given us several words in modern English, such as cosmology (the study of the universe), cosmogony (the study of the origins of the cosmos) and cosmetic. The last may seem somewhat surprising, but cosmeo meant not only to order or arrange, but also had a sense of good order and beautiful, aesthetically pleasing order." 
It is not.
If you write:
One of the reasons why we can say the ancient Greeks originated science is that they rejected explanation by myth, and began to explain using theories and began to make progress by debating and improving upon those theories.
That is plagiarism. If you write:
One of the reasons why we can say the ancient Greeks originated science is that they rejected explanation by myth, and began to explain using theories and began to make progress by debating and improving upon those theories. 
It is not. Even better, put 'as Gregory argues' before or after your summary, still using a footnote to give page details, etc.
Use footnotes (or endnotes), with short references to the titles - e.g. Gregory (2000) - and have a bibliography. For all works referred to:
Author, date of publication, title, publisher, place of publication.
A.D. Gregory (2000), Eureka ! The Birth of Science, Icon, London.
If you want to use a quote at second hand - so you see a quote in my book you want to use - you say 'quoted from' in your footnote. Only cite the source for the quote if you have gone to the source !
Web sources - quote the URL and the date you accessed the site (as the contents of pages are liable to change). You may quote my lecture notes, treat them as a web source.
Quotations should be short, and MUST be in English.
Use quotations to establish a point (X's view is thatâ€¦), or to introduce an important phrase (Einstein said "God does not play dice"), or theory (Special Relativity is based on two basic principlesâ€¦).
Do not use quotations to explain - that is your job !
Get used to this early !
Get used to giving good footnotes and a good bibliography early.
You lose marks if you do not, and you will carry on losing marks.
We will carry on penalising you for poor footnotes and bibliography.
No footnotes/ bibliography means one of two things:
1) You have not made use of the required readings. Or: 2) You have plagiarised.
Neither are acceptable.
Websites are not a substitute for the required readings.
Beware that websites are not always accurate.
Beware that websites often to not work at the level of sophistication required in University work.
There are good web materials, usually associated with Universities or other official bodies.
Wikipedia is not a good source !
Why do students plagiarise ?
Not realising what is required in terms of referencing and bibliography.
You have been told ! In general, get to grips early with what is required in University essays.
Plan a work schedule. If you are running late, talk to your personal tutor/ course tutor. Better a late essay, or a weak essay, or no essay at all, than to plagiarise.
Those for whom English is a second language may speak good English but be weaker at writing good English. Producing good essays may then take an inordinate amount of time.
There are English language courses available to help with written English.
Also beware - some students manage writing essays, by taking their time, but beware the exams where you will need to be able to write quickly.