Citation is foundational to research, publication, and information literacy.
Citation is the referencing of other researcher/writer's ideas so as to provide traces for future researchers to investigate, verify, or explore. It is also the prime way of preventing plagiarism by differentiating between another researcher's ideas and arguments from your own. Proper citation practices put you into a larger scholarly conversation on the topic.
Cite (document) any ideas or direct quotes used.
Cite when you paraphrase (using concepts and quotes indirectly).
Cite terms unique to an author’s research.
Cite items when you include textual facts, numeric data, and/or visual information.
Do Not Cite: proverbs, cliche quotes, and common knowledge (examples: Barack Obama was the 44th U.S. President; Thomas Edison invented the phonograph; "Mona Lisa" was painted by Leonardo da Vinci; Hydrogen has the atomic number 1). For determining “common knowledge”, consider if a skilled researcher would need to verify. When in doubt, cite the item to be safe.
Note: Find out which citation style is required for your discipline or subject (ALA, MLA, CMS, etc.).