Zotero's Web Library is where you sync your citations, notes, and if you have it setup to do so, your documents and/or PDFs that are attached to your citations. The benefit of syncing them to the Web Library is that you can now access your information on different computers including your mobile phone and/or tablet.
Zotero's Web Library has the majority of features that the standalone desktop software does with a few exceptions. There are no plugins available for the Web Library and the Related link option discussed in "Using Zotero" is not operational at the time of this guide's creation. However, you are still able to attach PDFs, notes, add tags, and even use the browser extension to save citations to your library. This page will show you how to use the basics in Zotero's Web Library.
To begin with, you still need to go to www.zotero.org and click the log in link at the top right of the screen.
Once you are logged in, it should immediately take you to your Web Library. If it does not, you should see a link across the top of the web page for "Web Library."
You will notice that many of the same basic features are available in the Web Library that are in the desktop version.
When you select a source in your Web Library, the information for that source along with notes, tags, attachments, and related sources will be editable to the right of the screen. As said earlier, you can do everything in the Web Library that you can do in the desktop version except link together related items, but if you have already linked them using the desktop version, that link will appear and work in the Web Library - you just cannot add new links for related items.
Since Zotero does not have a mobile app at this time, the best way to access your collection of sources on a mobile device is via the Web Library. The Web Library is made with a responsive design, meaning the layout and navigation changes depending on the screen size. As you can see from the images below, the Web Library has exactly the same options on a mobile device as it does on a regular computer but in a more compact layout intended for using fingers rather than a cursor.