Plagiarism is copying someone else's work and pretending you wrote it or created it.
Plagiarism is actually stealing, and is regarded very seriously at university.
This is a great example by Anne Robinson of the Dixie Grammar School Library, which teaches about research, plagiarism and citing references.
(Permission given for re-use)
This video about fair use from the Temple Media Education Lab specifically relates to the USA, however it still has very useful guidelines for everyone.