A citation is the evidence that you have done research. Citations link your reader to the sources that informed your thinking and show that you are participating in a scholarly conversation. Offering a citation gives your readers and other scholars access to your information sources if they wish to follow-up, or find more information on your topic.
Why should I cite my sources?
To demonstrate you have considered others' research
To direct readers to additional information
To establish credibility as a careful researcher
To show professional honesty and courtesy
To avoid issues with plagiarism
What about 'common knowledge'? Are those cited?
Common knowledge includes facts that are known by a lot of people and can be found in many sources. An example might be the boiling or freezing temperature of water.
Common knowledge can vary between subject fields, so think about your audience.
If you have doubts about whether something is common knowledge, ask your instructor or another expert in the discipline.