Most basically, plagiarism is cheating. It’s laying claim to thoughts, ideas, facts that aren’t your own.
It’s something that can be done deliberately (e.g. by downloading others’ work from Internet essay banks).
It’s also something that can be done without intent (e.g. some students, not knowing quite what constitutes plagiarism, can inadvertently fall into it). This will still be plagiarism, though: still unacceptable and, in many cases, subject to sanctions.
In-between these two extremes – of deliberate, obvious plagiarism and inadvertent plagiarism – comes other shades of the same practice.
Sometimes student writers blend their own writing with those of others (e.g. texts they have read) but fail to acknowledge their sources (that is, they don’t say who’s written the source texts – they fail to give the appropriate bibliographical referencing).
Such a practice is plagiarism again.
Have a look through the ‘How to…’ guide’s section on plagiarism.
If you’re worried about referencing go to the JISCPas site and continue reading through this suite of pages.