When Does General Information Need to be Cited?
One gray area that deserves special attention: where do you one draw the line between material that is widely known and thus does not need to be cited, and material that does need to be cited?
Here's a principle to work with: information that can be found in five or more different sources can be considered general information and thus does not need to be cited.
You need not document the fact that Einstein presented his general theory of relativity in 1916, because this fact is widely known and published. On the other hand, you would document little-known information on the contemporary sources of his theory (perhaps found in a biography of Einstein), and you would document another scientist's opinion of Einstein's theory (for example, Stephen Hawking's in A Brief History of Time).
If you are still uncertain about what plagiarism is and isn't, consult any college English handbook or guide to the writing of research papers. Nearly all of these sources feature concrete examples of the do's and don'ts of integrating someone else's words and ideas.