In case you are quoting words that were said by someone else, you must cite him or her in a research essay.
In case you are paraphrasing a statement, idea, or thoughts by someone else in your own words, then you must cite him or her in your research essay.
In case you are summarizing a statement, idea, or thought by someone else, then you have to cite him or her in your research essay.
Also, you must cite facts, information, and data that you have sourced from other sources.