Your Zotero citation database is stored on your hard drive. However, if you want to add to it when you are not at your Zotero computer, you can use a free Zotero account that can automatically sync with your computer(s).
To sign up for the free account, go to the Zotero site at https://www.zotero.org and click the link in the upper right-hand corner, "Log In." On the next screen, choose "Register for a free account." You do not need to have a Zotero account to use Zotero, only to sync and share.
Fill in the form, then check your email for the register confirmation. Then return to the Zotero site and log in by clicking the the link in the upper right-hand corner, "Log In."
Your online Zotero account uses tabbed navigation at the top of the screen.
The "My Library" tab can be synced to your citation database.
The "Groups" tab in the menu allows you to create or join groups for collaborative work. You can control who the group's members, and what they can do with the citations records, such as viewing, adding, or deleting.
Once you have the free, online, synced Zotero account, you will want to set up the auto-sync feature in the Zotero client. Click on Edit>Preferences>Sync.
Enter your new online Zotero username and password in the fields provided. Click the "Set Up Syncing" button. Make sure the "Sync automatically" box is checked.
You must decide if you want to sync your attached files (PDFs, audio, video, etc.). You may sync an unlimited amount of citations, notes, and tags for free. However, you have only 300MB of attached file storage, which is called Zotero File Storage. Additional space in Zotero File Storage for attached files may be purchased at an additional cost.
Syncing is not considered to a secure method of backing up your Zotero database. Errors can occur and data can be lost in a large sync. Zotero recommends another method for safely backing up your citations, and it is important to regularly protect yourself from a harddrive crash or a virus. This procedure also works if you want to make your Zotero citation database more portable on multiple computers.
First, create a desktop directory with a useful name, such as Zotero Back-Up yyyy-mm-dd.
In your Zotero window, click Edit>Preferences. Click on the advanced tab.
Click the "Files and Folders" tab.
To back up your entire Zotero directory, with database and attached files, click the "Show Data Directory" button.
Ctrl+A to highlight everything in the directory. Copy (Ctrl+C) these files and folders. Close all windows. Open the back-up folder. Paste (Ctrl+V) all the files and folders into the back-up folder (Zotero Back-Up yyyy-mm-dd).
You have now backed-up your Zotero directory, including all citations, notes, tags, and full-text PDFs. Move the back-up folder to Dropbox, Google Drive, or a USB drive. Do not keep it on your harddrive.
You can also use this USB and back-up as a portable Zotero database on another computer with Zotero installed. Under Edit>Preferences>Advanced click the "Custom" radio button, and use the Choose button to select your USB drive and the Zotero directory.