You should cite any information you got from reading other people's work. This includes:
• Direct quotations
• Information you summarize or paraphrase from your sources
• Any references to the work of others. Keep an eye out for phrases like "Some people think" or "Researchers have said"
A good general rule is to cite any information you didn't know before you started doing research.
A citation is not needed for common knowledge, widely known facts and ideas and your own thoughts.
(From Portland State University)