Plagiarism is the unacknowledged use of other people's for work as your own, and is considered cheating. The following are forms of plagiarism:

Using phrases or sentences verbatim, i.e. copied exactly, from an author without any acknowledgment by means of quotation marks and reference.
Paraphrasing the ideas of an author without acknowledgment.
Copying, or appropriating the work of another student.
Submitting the same piece of work in more than one subject without obtaining prior permission from the staff involved.

Plagiarism is a serious offence and all university regulations clearly prohibit its practice. Penalties rang from receiving a mark of zero right through to expulsion (depending on faculty policy). One particular case that comes to mind is that of a student who plagiarised an entire document, and upon submitting it for correction discovered that the work he had copied was in fact written by the very same person who was to correct it. Needless to say, that student was caught out and thrown out of school.

When in doubt it is best to include a reference or seek further assistance from lecturers.