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)
The MLA style is generally used in literature, arts, and humanities.