Much of what we know of the Rutans, or as they are more properly known The Rutan Host, comes from the stories featuring the Sontarans. The two races have been in conflict for millions of years.
The Rutan Host appear to be a communistic race. They reproduce via binary fusion, creating identical duplicates. Though they refer to themselves as We, Us, and Our(s) they function independently. The Rutan hive mind is co-ordinated by a Queen who lives on their home planet Ruta III, without her influence the Host would collapse.
Their true appearance resembles a glowing green blob, much like a jelly fish with small tentacles. Being amphibious they can move easily through water. Though they seem to prefer colder climates but can also adapt to any terrain, moving up a cliff face or in the vacuum of space. They can generate bio-electrical power to shock their victims, who they can then morph into, using advanced shape shifting technology to assume the form of their prey.
The Doctor has only ever encountered the creatures once during his travels through time and space, and that was during the life of his fourth self. During the adventure Horror of Fang Rock he discovered that the creature which had infiltrated and murdered the inhabitants of a lighthouse was in fact a Rutan.
The Doctor himself once remarked that the Sontaran's and the Rutan's have been fighting for so long now that they've probably forgotten what started it!
However don't be fooled by a Rutan's natural appearance, they are a formidable enemy, a sentiment which every Sontaran would tell you. Rutan's tend to infiltrate other alien cultures, kill one of them and then assume it's identity. They are covert spies and have masterminded their technique over millennia.