A citation is a listing of all the information necessary to identify and locate a specific published source - whether it's a book, an article, a video, a website, or a tweet.
Citing your sources is important for three reasons:
When you find information in another source, whether it is a newspaper, magazine, academic journal, or even online, someone else has published it, which means that essentially that person "owns" the information and the ideas (intellectual property). Not giving that person credit when you borrow their ideas or words is called plagiarism, and that is a very serious academic infraction.
(http://guides.library.illinois.edu/citingsources - Undergraduate Library, University of Illinois at Urbana-Champaign)
Most of the library databases have a citation tool that will create a citation for an item. When using the citation tool, make sure you choose the style that your instructor requires (MLA, APA, Chicago). Be aware that while these database-generated citations are a fine place to start, you will need to check the details of the citation using a style guide.
Consider these tips when using citations from a database.