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.
Direct plagiariam - copying something word for word without citing the source.
Self-plagiarism - re-using an assignment you wrote for another class.
Accidental or unintentional plagiarism - not citing your sources properly.
Collusion - claiming credit for work someone else did for you.
(Information from Plagiarism, helios.weber.edu/orig/menu.htm)
This video about fair use from the Temple Media Education Lab specifically relates to the USA, however it still has very useful gudelines for everyone.
This is a humorous compilation of Disney clips, outlining the basics of copyright.