Zotero's online syncing allows you to access your Zotero library on any computer with internet access. Zotero syncing has two parts: data syncing and file syncing.
Data syncing merges library items, notes, links, tags, etc.—everything except attachment files—between your local computer and the Zotero servers, allowing you to work with your data from any computer with Zotero installed. It also allows you to view your library online on zotero.org. Data syncing is free, has no storage limit, and can be used without file syncing.
The first step to syncing your Zotero library is to create a Zotero account (which is also used for the Zotero Forums). Then, open Zotero's Sync preferences tab and enter your login information in the Zotero Sync Server section. By default, Zotero will sync your data with the server whenever changes are made. To disable automatic syncing, uncheck the “Sync automatically” checkbox in this section.
The first time your Zotero library syncs, Firefox may appear to hang, especially if you have a large library. Be patient. If the Zotero sync icon continues to spin and you're convinced there is a problem, see Sync Queuing.
By default, Zotero will merge your local Zotero library with your library on zotero.org—any changes you make in one place will be applied to the other and on all other synced computers. If an item has changed in multiple places between syncs, you'll receive a conflict resolution dialog asking which version you'd like to keep. In the rare case that you want to completely overwrite your server library with the contents of your local library or vice versa, you can use the Sync Reset Options.
Data syncing syncs library items, but doesn't sync attached files (PDFs, audio and video files, images, etc.). To sync these files, you can set up file syncing to accompany data syncing, using either Zotero File Storage or WebDAV.
Zotero File Storage is the recommended file sync option and has several advantages over WebDAV syncing, including syncing of files in group libraries, web-based access to PDFs and other attachments, easier setup, guaranteed compatibility, and improved upload performance for certain files. Each Zotero user is given 300 MB of free Zotero File Storage for attached files, with larger storage plans available for purchase.
See the Zotero File Storage documentation for more information.
WebDAV is a standard protocol for transferring files over the web, and it can be used to sync files in your personal library. (Group libraries cannot currently use WebDAV.) Your employer or research institution may be able to provide WebDAV storage. Otherwise, there are many third-party options, both free and paid (see WebDAV providers known to work with Zotero).
Once you have your WebDAV account info, enter the URL provided by the service, your username, and your password in the Sync preferences tab. Be sure to select 'http' or 'https' as appropriate — if you're not sure, try 'https' first. After entering the information, click “Verify Server”. If Zotero successfully verifies the WebDAV account, you're all set to use file syncing via WebDAV.
Zotero file sync should work with any correctly functioning WebDAV server. Zotero developers cannot provide support for third-party WebDAV servers.
If Zotero is set to sync automatically, changes will be synced within a few seconds of being made. Otherwise, you can start a manual sync by clicking the sync button on the right-hand side of the Zotero toolbar.
If you enter the same login information into the Sync preferences on multiple computers, Zotero will sync everything transparently. Your attention should only be needed if the same item or file is edited in two different places before Zotero has a chance to sync them. In this instance, you will be presented with a conflict resolution window, where you can decide which changes to accept.
If you sync from only one computer, you can still view your online library at zotero.org from any computer. Should something happen to your computer or should you want to start using Zotero on another computer, simply set up your account info on the new computer. Zotero will pull down your entire library from the server.
If, for whatever reason, you are unable to use Zotero's syncing features, you can opt for several alternative syncing solutions, such as running Zotero from Portable Firefox or syncing Zotero data using a network folder, a folder synchronization service, or a tool such as rsync. Note that none of these solutions are recommended or supported. While using a third-party tool to sync the 'storage' subdirectory of the Zotero data directory is generally safe, syncing the entire Zotero data directory, including the main SQLite database, with an automated third-party tool is extremely likely to corrupt it. When using any of these options, make sure to back up your Zotero data frequently.
A key problem in using a folder synchronization service, such as Dropbox, is that Zotero allows you to change the location of the data directory but does not allow setting the location of the storage. There are two workarounds for this problem: 1) You can use a symbolic link to sync only the storage directory. 2) You can set the data directory inside Dropbox and then use selective sync options to disable syncing for the data directory and then enable syncing of the storage directory. The first option is probably more commonly used, but either one should work. But you should understand that if you set up the second option incorrectly, you risk losing data. The forums contain several threads about the problems that users are facing with Dropbox-based setups.