Whenever you use an outside resource in your work-- whether you are quoting, referring, or appropriating-- you must give credit. Why?
Scholarship is a conversation: Good work doesn't spring fully formed from your brain -- it is the end result of critical engagement with the ideas and work of others. You should cite all books, articles, websites, images, etc. that directly inform your work, just as you would want others to give credit to what you create. Properly formatted citations in an accepted style (MLA, Chicago, APA, etc.) are essential to maintaining artistic and scholarly conversations.
It is required: Tufts' Academic Integrity Policy specifically prohibits plagiarism. A key way to avoid plagiarising the work of others is to carefully cite all outside references. Your instructors have most likely indicated which citation style they want you to use, and provided some basic guidelines. See below for some additional resources that will help you format your citations.
The Purdue University OWL (Online Writing Lab) guides are a fantastic resource for quick overviews of the various flavors of citation styles. They include bibliography and in-text citation formatting examples for a range of source types (books, articles, lectures, websites, online videos, sound recordings, and much more).
See the Tisch Library's Citing Sources guide for an exhaustive overview of citation styles, guides, and options for managing your bibliographies.