Mendeley is online software for use in citation management - that is, keeping your cited references (books, articles, etc.) organized and properly formatted so they can be included in your finished paper.
Important features and things you need to know about Mendeley:
Want to learn more about different citation management software? Take a look at this chart (note that features of different programs change all the time; we do our best to keep our information current, but be sure to look at the documentation and websites for these programs themselves to be sure).
Zotero is another citation manager. Unlike Mendeley's basic install, it allows for unlimited number of peoples to use group folders. A Zotero free account gives users a base allotment of 300 MB of storage, with further storage available for a fee.
More details on Zotero, including installation, troubleshooting and use can be found on this page here.