Using someone else's work without citation is plagiarism and may have serious consequences for students and scholars.
Any time you incorporate someone else's ideas or words into your paper or presentation, you must cite them. These ideas may not only come from books and journal articles, but from conversations, images, multimedia, speeches, blogs, etc. They need to be cited for several reasons:
For general guidance, see the ODU Libraries guide to citing sources (linked below). If you're unsure, ask your professor which style s/he prefers.