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A citation is a brief summary of a published work that provides all the necessary data about a book, journal, or website so that someone else can easily find this same source on their own.
There are standard formats for citing different types of source materials, like journals, books, and websites. Generally a citation may include all or part of the following pieces of information:
Title of the work: article, book chapter, book, technical report
Publisher of the work
Volume, issue, or edition of the work
URL or DOI (“A digital object identifier (DOI) is a unique alphanumeric string assigned by a registration agency (the International DOI Foundation) to identify content and provide a persistent link to its location on the Internet. The publisher assigns a DOI when your article is published and made available electronically.” http://www.apastyle.org/learn/faqs/what-is-doi.aspx)
When conducting a literature review, you may gather a variety of citations to journals, books, datasets, technical reports, and professional guidelines. Keeping track of all these resources/citations doesn't have to be overwhelming. Learn how to use bibliographic management software like EndNote, Zotero, or Mendeley. Let the software easily capture your citations and create the proper formatting for you. While you are welcome to use any software you think is best. The library officially supports EndNote but librarians are also knowledgeable about the other two.