In the article rsync server setup on rhel, I've described how to setup rsync server, now here is a bit more instructions for who wants to use rsync with ssh.
Basically, it's the same way you setup ssh key authentication, so good part is that your rsync process will be performed via secured connection, but bad part is that there is potential security danger. Anyway, in most cases, rsync and ssh key setup are mostly setup internal network, so best leave it to you to judge.
# ssh-keygen -f ~/.ssh/id_rsa -q -P ""
# cat ~/.ssh/id_rsa.pub
copy it to a file or clipeboard, then on server
add it to ~/.ssh/authorized_keys
cat id_rsa.pub >>~/.ssh/authorized_keys
# rsync -auvx -e 'ssh' testrsyncserver::backups/ .
receiving incremental file list
sent 36 bytes received 56 bytes 61.33 bytes/sec
total size is 0 speedup is 0.00
You are all done.