If an idea or fact is widely know and not disputed, it is referred to as common knowledge and does not need to be cited. This information is generally known by everyone within the discipline and can be found in numerous sources.
Examples of common knowledge:
It is important to understand the distinction between common and specialized knowledge. If you're not sure whether a given idea counts as common knowledge, aways err on the side of caution: When in doubt, cite it.
Examples of specialized knowledge:
Reusing your work from one class in another class is known as "self-plagiarism."
Why does self-plagiarism matter?
Say you submit a paper for a class and receive a good grade. Next semester you submit that same paper again for a different class and receive another grade.
You just received two grades, but you only did the work once. Self-plagiarism is essentially a form of dishonesty.