Plagiarism

Plagiarism can be simply defined as "a piece of writing that has been copied from someone else and is being presented as your own work". That's not my definition", I've copied it from someone else (from here), which is why I've put it in quotation marks and given the link to where it appears ( a nice online dictionary named Wordnet).
There's often confusion over what constitutes plagiarism, but it's quite simple - see the definition above. I'd add a couple of words to that definition. At the end of the definition, I'd add "even inadvertently". We sometimes see assignments in which some text has been lifted word for word from another source, but the source is referenced in the bibliography. This, I'm afraid still qualifies as plagiarism. Why? Two reasons:
When you use the exact words of another, you must use quotation marks, otherwise the reader is led to believe that those words are your own. Secondly, there's no link between the text and the source; without any indication of where the non-original text comes from, the use of quotation marks is meaningless. Even if you're paraphrasing someone else's words
Hence in my definition of plagiarism above, I put the non-original text in quotation marks and I gave a link to the source of the quotation. When writing assignments, you need to put the source of any non-original text beside that text. You don't need to put the entire reference, the author's surname and year of publication will normally suffice. If you're unsure about referencing, why not take one of the library's great referencing classes?