Remember to regularly back up your Zotero library. Don't rely on online syncing. Save a back up file to a USB or external storage device. Do not save your back up to cloud storage.
To make a back up of your Zotero library, make a copy of your Zotero data directory.
This is located in Zotero Preferences > Advanced > Files and Folders > Show Data Directory.
To create a back up, close Zotero* and copy everything in this folder location. Then save to a storage device. Your PDFs (if you have any saved to your Zotero library) will be in the data directory in a folder labeled "Storage".
* Warning: Before you copy, delete, or move any of these files, be sure that Zotero is closed. Failure to do so before moving these files can damage your data.
While Zotero stores all data locally on your computer by default, Zotero's sync functionality 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 and to view your library online on zotero.org. Data syncing is free and unlimited, and it 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 the Sync pane of the Zotero preferences and enter your login information in the Data Syncing section.
By default, Zotero will sync your local data with the Zotero servers whenever changes are made. To disable automatic syncing, uncheck the “Sync automatically” checkbox in this section. You can sync manually at any time by clicking the “Sync with zotero.org” button on the right-hand side of the Zotero toolbar.
When Zotero syncs, it automatically applies changes in both directions — any changes you make in one place will be applied to all other synced computers. If an item has changed in multiple places in conflicting ways between syncs, you'll receive a conflict resolution dialog asking which version you'd like to keep. If you find yourself using a new computer, you can simply set up syncing and Zotero will automatically download all data from your online library.
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 Storage or WebDAV.
Zotero groups provide a powerful way to share collections with a class, work closely with colleagues on a project, keep track of conversations in your field more broadly, and keep tabs on what people at your institution or in your department are working on. There is no limit on how many members may join your groups, and your full storage subscription is always available to your personal and group libraries.
Group Libraries are separate from your personal My Library, so you can also use Zotero groups as a way to keep separate libraries within one Zotero profile.
More information can be found on the Zotero guide.
