Zotero is not a website, but lives on your computer, connected to your web browser. If your hard drive dies, you could lose all of the material collected in your Zotero Library if you don't back up your data!
Backup by exporting your Library periodically. Then save the resulting folder to an external hard drive, a web-based system like Dropbox, or wherever you backup your other files.
Select: 
> Export Library...
(tick the boxes to include Notes and Files)
Create a free account at zotero.org. Then enter the same credentials in your local Zotero Library.
Select: > Preferences > Sync
Zotero will then automatically sync your local Library with their server.
There is no limit to the number of citations that can be synced, but there is a cap on the free file storage (attached pdf's), though more space can be purchased.
Syncing is the preferred backup method because it will automatically protect your data. But, there are other benefits to syncing with zotero.org -- it allows you to:
