Any material that comes from any source other than your own mind MUST be cited. This includes information gleaned from the textbook or classnotes. Common knowledge need not be cited, but there is often some debate about what is common knowledge. Operate from the assumption that your reader might not have the common knowledge you have even if you are writing for a specialist such as myself. Finally, there is a way of constructing a paper that I consider a type of plagiarism.
Some papers consist of a series of quotations linked together by a few sentences or paragraphs.
While the quoted material is cited and this is not technically plagiarism, it is still unacceptable. Even if you are not claiming the words or ideas as your own, you are allowing another author to write your paper. Any student who turns in a paper with more than 2 or 3 direct quotations per page is guilty of intellectual colonialism. Have the courage of your own ideas. I am capable of reading the works you are quoting.