If you're regularly using more than one computer in your research, Zotero's sync feature can keep your library up to date on all of them. Zotero can store a copy of your library on the Zotero.org server and check it for updates whenever you open your library on a different computer. All your computers must be running the same version Zotero and be configured to sync to the server.
First, set up a (free, of course) Zotero.org user account.
Open Zotero preferences (via the gear menu) and select the Sync tab. Enter your Zotero user name and password. Check the "sync automatically" box. Zotero will upload your library to the server.
Repeat this configuration on each of your computers. Any updates you make on one of your computers will be reflected on the others. This even works to synchronize your library among Windows, Mac and Linux computers.
For more details and help troubleshooting sync problems, check the Zotero site.
It's easy to add PDFs to your Zotero library and automatically import their citation info.
First, enable PDF indexing on the Search tab of Zotero's preferences. Zotero will download and install a small plugin.
Next, just drag your PDF files into the Zotero pane.
Right-click the PDFs and choose "Retrieve Metadata for PDFs." Zotero will retrieve their citation data from Google Scholar and turn them into citeable items with PDF attachments.
If Zotero can't find a match on Google Scholar, don't worry -- you can still save the citation from another catalog or article database, then drag the PDF onto the citation to make it an attachment.
Have the book in front of you and want to add it to your Zotero library without having to search for a citation?
If you have a book's ISBN, an online article's DOI or PMID number, just click the magic wand button: "Add item by identifier." Type in the book or article's number, and Zotero will automatically download its information and save it to your library.