This is different than a rimjob. A rimjob would, however, be a precursor to anilingus. It is the act of plunging one's tongue into the
anal cavity of your
sexual partner and then swirling and
thrusting aforementioned tongue in an effort to make said partner happy.
It is well advised to wash the anus that is to be pleasured thoroughly (and perhaps even douche) before attempting this.