Judoon are kind of like an intergalactic police force, dedicated to wasting everyones time by marching to places, and cataloging every life form in the whole universe.
They just look like giant rhinos in suits. They have a rivalry with the Sontarans, but then again, who doesn't?
They also have a shaky relationship with the Doctor , helping him occasionally but also fighting him sometimes.
They also apparently have their own song...