Create backup user?

I want to create a user for the sole purpose of pulling backups via Rsync to a NAS on my lan. I have a FreeNAS box running on my LAN, and my CentOS server does a daily backup to a local directory (~/backup).

I want to pull in each daily backup from the CentOS to my FreeNAS box.

I suppose I could do this as root, but I figured it makes more sense to make dedicated backup user that can only read the contents of the /backup directory.

My question is: How do I create a user that only has access to /backup?