Creating an account on the Zotero Web site allows you to have a copy of your library on the Zotero server, very convenient if you are not always working on the same computer or for collaborative work.
After creating your account, enter your username and password in your Zotero preferences: select Preferences in the Edit menu, then Sync., enter your username and password. You have to repeat this configuration on each of your computers. The synchronisation is then done automatically after each change in your database.
You can sync an unlimited number of references, notes, and collections. PDFs and other attachments take up storage place, which is limited to 300 MB for free. You can purchase additional storage space if necessary.