Citation Managers are tools to help you keep track of your citations as you research and to create/format your citations and bibliography. For example, Zotero allows you to keep citations, full text articles, and other research resources organized in one place. You can also use these tools to format your bibliographies and the notes/citations in your papers according to the appropriate style (APA, MLA, Chicago, etc.).
Zotero is a free open source application that works with Firefox, Chrome, and Safari, you can also download a client to your computer. It is a good choice if you use just one computer when researching. You can access citation information when online or offline.