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:
Anything that you read in any format like books, journal articles, web pages, etc.
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.
When writing, think about your reader's perspective. Don't assume that if you know something, everyone else knows it too. So for example, let's say that you're really into tattoos and you decide to write a paper about tattoos and society for your class. If you use the term "inking," you should explain this term (or concept) and probably cite your source since your audience might not be as familiar with the lingo. Also, citing the source will demonstrate that you know what you're talking about and not just making up new words. On the other hand, if you were writing an article for Tattoomagazine, a publication for tattoo enthusiasts, your audience might not need the explanation because they would be in the know.
Harris, Leigh. Personal Interview. 30 July 2003.