Cite the sources of all ideas and information that are not your own and that are not common knowledge. All ideas and information taken from a source must be acknowledged unless they are considered common knowledge. The crucial term in this rule is, of course, common knowledge.
There is a simple test to determine whether something should be considered common knowledge and need not be documented: Would this idea or piece of information be familiar to someone like you (a classmate, for example) who has not researched the subject? If the answer is yes, then you do not have to cite the source. Otherwise, you must indicate the source of the material, even if it appears in several texts. Contrary to some commonly held opinions, that an item of information appears in several sources does not make it common knowledge. Some of the most obscure facts in physics or biochemistry, for example, will appear in numerous articles or texts over the years. Still, they are hardly common knowledge.