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. In Zotero, each of these references constitutes an item. Every item contains different metadata, depending on what type it is. Items can be everything from books, articles, and documents to web pages,artwork,films, sound recordings, bills, cases, or statutes, among many others.
Zotero can add items automatically using ISBN number, Digital Object Identifier (DOI) or PubMed ID. This is done by clicking the Add Item by Identifier button in the Zotero toolbar, typing in the ID number and clicking OK. Items synced to the Zotero server can be accessed online through your zotero.org account. Share your library with others or create a custom C.V. from selected items.
Zotero users can create collaborative or interest groups. Shared group libraries make it possible to collaboratively manage research sources and materials, both online and through the Zotero client. Zotero.org can be the hub of all your project group's research, communication and organization."
A few noteworthy Zotero reviews: from Wired, Chronicle of Higher Ed (ProfHacker compares EndNote and Zotero)
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