$ rsync -a a b <==> b/a/foo
$ rsync -a a/ b <==> b/foo
destination (b) <==> destination/ (b/)

If you need to pass parameters to rsync-over-ssh, then includ them in ‘ - as in:

$ rsync -ave ’ssh -i /home/netfortius/.ssh/remote_private_dsa’ user_at_remote@remote_system:/path/ ./local/path