Always cite other people's words, ideas and other intellectual property that you use in your papers or that influence your ideas. This includes but isn't limited to:
You don't need to cite what would be considered common knowledge., such as facts, events, concepts, etc. that are widely known and accepted as true.
For example if you wrote, "President Zachary Taylor died in office," this wouldn't need to be cited because it's an accepted fact, or common knowledge.
BUT, you should cite something that is controversial or contradicts what most accept as common knowledge. For example if you assert that President Zachary Taylor died in office from eating a bowl of contaminated cherries, or that he was poisoned, you should cite your source of information because neither are generally accepted as fact.
Leerhsen, Charles, et al. "A Tale of Arsenic and Old Zach: Was He Murdered in the White House? Or, Did Old Rough and Ready Die of One Cherry Too Many?" Newsweek, 1 July 1991, pp. 64-66.
Need more? See Resources for UCLA Students.