If you are quoting or paraphrasing information, reusing an image, restating information presented in a table, or if you otherwise did not think of the information you are using, a source should be cited. Citations give authors/creators/researchers credit for their work.
Failure to properly cite sources is considered plagiarism, whether intentional or not. Plagiarism is:
Keep track of your sources and cite everything.
If you’re not sure whether something needs to be cited, ask your instructor or a librarian.
Make sure you cite even when you paraphrase – not just when you use direct quotes.
When you do paraphrase, use your own words. Don’t just rearrange sentences or replace select words. You need to show that you understand this information and are using it for a reason.
The UC San Diego Libraries' Avoid Plagiarism video gives an excellent overview of how to avoid plagiarism.