Syncing your library
Syncing your library backs up your library on the Zotero server, and enables you to use Zotero on more than one computer.
After you set up up a free Zotero.org user account, open the Zotero Preferences window and navigate to the Sync tab. Enter your Zotero.org username and password.
Check the "sync automatically" box, and check both boxes under File Syncing and choose Zotero storage for My Library. This will sync your PDF attachments as well as citations (more info).
Zotero will upload your library to their server.
In addition to automatic syncing, you can sync manually at any time by clicking the “Sync with Zotero Server” button on the right-hand side of the Zotero toolbar.
If you own more than one computer, repeat this configuration on each of your computers. Any updates you make on one of your computers will be reflected on the others.
For more details and help troubleshooting sync problems, check the Zotero site.