There are at least three popular citation managers. EndNote, from Thompson Reuters, was considered the standard until the development of free alternatives, including Zotero and Mendeley, that also provide robust functionality.
EndNote: software for publishing and managing bibliographies, citations and references. The fully featured desktop EndNote software requires a subscription. EndNote Basic is a free online version to track resources and generate bibliographies but has limited functionality. Thee library has a guide on how to use EndNote.
Zotero: free software tool to download or use online to collect, organize, cite, and share research sources. Zotero is preferable if your research content is varied as it provides an easy capture method for non-pdf content. Storage is unlimited on one’s personal computer, but free online storage is limited to 300 MB.
Mendeley: another free, robust citation manager that works best when your research resources are mostly pdf files. With Mendeley’s pdf viewer, you can import, view, and generate citation records from pdf files. It also offers 2 GB free online file storage.