Whenever you are using a source that is not your own words, you need to cite that source. Citation styles can vary (APA, MLA, Chicago/Turabian, or others) and are course and discipline-specific. Get into the habit of citing your sources at all times. Take a look at Chapter 5, “Academic Integrity and Documentation,” which covers citations in more detail.
Understanding why writers use citations in academic research can help you decide when to use them. Acknowledging intellectual ownership shows respect for those who have contributed to the field of knowledge and for the achievements in that field, as well as how you are contributing to it. Engaging in that field in meaningful ways is an important part of your learning journey. Citing reliable sources gives your research and writing credibility, showing your familiarity with the work of a scholarly community and your understanding of how you are contributing to it. Finally, it is unethical and illegal to use the ideas and words of others without acknowledging and recognizing those who own the intellectual property rights to those ideas and words.