The ip domain-name command assigns the host (device) a domain name to more precisely identify it. The FQDN (hostname + domain-name) is used by the device when you generate a local RSA host key which is a prerequisite for running secure shell (ssh) as it is used to encrypt the traffic.
The "-l" is simply a way to specify the username to use when initiating an ssh client session from the device. See the command reference here. (By the way on NX-OS we use "ssh username@server".)