There's manual syncing, and automatic syncing. Manual: All clients I've seen have the capability to merge databases. So you have one copy of the database in wh

There's manual syncing, and automatic syncing. Manual: All clients I've seen have the capability to merge databases. So you have one copy of the database in whatever online file storage service, plus each device will have its own local copy. Pull down the online db, do a bidirectional sync between the two databases, push back online. Automatic: some clients natively support webdav, dropbox, etc as the master copy of the db file and will transparently do the change syncing for you.