Plagiarism is a term given to a particular type of lack of academic integrity. It is about taking and using another person’s work and claiming it, directly or indirectly, as your own. ‘Work’ means anything that has been written by another, including published work or work that has been presented publicly in some way, including the internet. It also includes work written by another student, so would include buying a ready written assignment from an internet source, or elsewhere, and presenting it as your own work.

Plagiarism can sometimes be unintentional, especially where students’ previous educational experiences have actively encouraged the compiling of material from outside sources as an approach to writing essays. In higher education this would be regarded as plagiarism, unless the sources were properly acknowledged and the content was accompanied by a critical commentary.

• Copying or buying another person’s work and claiming or pretending it to be your own.
• Presenting arguments that use a blend of your own and the copied words of the original author without acknowledging the source. This has become a particular problem because of the ease of copy and paste from the Internet. However, plagiarism detection software can now highlight which parts of an assignment have been directly copied.
• Paraphrasing another person’s work, but not giving due acknowledgement to the original writer or organisation publishing the writing, including Internet sites.
• It also includes collusion with other students, for example, when one student writes an assignment that others copy (or buy) and then present individually as their own work.

It is a mistaken assumption that you can copy chunks of information from a source and simply add a citation to make it acceptable. It is not. Your assignment needs to be your work and simply copying in other people’s work, even if you cite the source, is not acceptable and is regarded as a form of plagiarism.

Whether or not you are quoting directly, or summarising or questioning ideas and information that have contributed to the development of your ideas, it is still important for you to give full details of your sources.

• Accurate referencing will help you to avoid any accusations of plagiarism.
• Applying, analysing, criticising or quoting other people’s work is perfectly reasonable and acceptable providing you always:
• Attempt to summarize or restate in your own words as far as possible another person’s work, theories or ideas and give acknowledgement to that person. This is usually done by citing your sources and presenting a list of references
• A certain amount of overlap between the words of an original author and a student’s own words is inevitable, particularly in descriptive areas, e.g. describing and naming organisations, institutions or other factual things. However, analytical features of a work should always be attempted by the student in his or her own words.

or :-

• By always using quotation marks (or indenting lengthy quotations in your text) to distinguish between the actual words of the writer and your own words. You would cite all sources and present full details of these in your list of references.
• Therefore, to avoid plagiarism, you must give credit whenever you use:
• another person’s idea, opinion, or theory;
• any facts, statistics, graphs, drawings—any pieces of information—that are not common knowledge;
• quotations of another person’s actual spoken or written words; or
• paraphrase of another person’s spoken or written words.

Some great advice on plagiarism and how to avoid it from University of East Anglia
How to avoid plagiarism