From the Zotero Quick Start page:

"Before learning what Zotero does, it is important to know
what it is. Zotero is an extension for the Firefox web-browser. It runs in its
own pane within Firefox, separately from web pages...
Zotero is, at the most basic level, a citation manager.
It is designed  to store, manage,  and cite bibliographic references, such as
books and articles, documents, web  pages,

artwork, films, sound recordings, bills, cases, or statutes, among many others. Items synced to the Zotero server can be accessed online through your zotero.org account."

• Get Zotero here!  (Works with Firefox, Chrome and Safari)
• Zotero is pronounced Zoh-TAIR-Oh. Begin with a tour of Zotero.
• Zotero support and instruction for Duke users is offered by Duke librarian Ciara Healy.
• Download Zotero word processor plug-ins for in text citations, to cite as you are writing your document.
• Export from EndNote or other citation managers.
• Check out Zotero's extensive library of screencast tutorials.

A few noteworthy Zotero reviews: from Wired, Chronicle of Higher Ed (ProfHacker compares EndNote and Zotero)