Plagiarism is a form of cheating. It essentially boils down to inappropriate use of information in your work or failing to provide an complete citation for material you have used, leading to the impression that it entirely your own creation.
It is often a result of poor referencing skills or not realising what you are doing is actually wrong.
Have a look at the links below, which explain what plagiarism IS and provide tips on how to avoid it.
The University of Ulster Plagiarism Policy (2012) defines plagiarism as:
"...the act of taking or copying someone else’s work, including another student’s, and presenting it as if it were one’s own...Plagiarism also occurs where a student’s own work is re-presented without being properly referenced. Plagiarism is a form of cheating and is a disciplinary offence."
You can find the Plagiarism Policy on the A-Z list of University Policies.
This video from Bolton University explains what plagiarism is and how you can avoid it.