Questions to ssh connection with keys

Hi,

I think of using sftp as transfer for backups and using keys instead of passwords. I understand it works this way:

I've got 2 servers: server1 and backup I want server1 to connect to backup and upload a file. So I do this on server1: ssh-keygen ssh-copy-id user@backup and then I can doo sftp user@backup without passwords.

Is that right? And when I do ssh-keygen what do I have to write as passphrase? And what should I save on my computer as backup? The identification public key, key fingerprint and key's randomart image?

Regards and thank you :)