An exoskeleton was, according to River Song, "basically life support." It was able to protect the user from any or most environmental hazards and prolong their life by providing them with nutrition and other biologically necessary chemicals. Such suits could also be weaponised by enhancing effort and exertion, and therefore strength and resilience, to inhuman levels or the suit having built in weapons, usually energy weapons, as River Song's space suit was.
One such exoskeleton, in the form of a space suit, was owned by the Silence, and had about twenty different kinds of alien technology in it. River claimed that, with it on, you didn't even need to eat. The exoskeleton processed sunlight directly and had built-in weaponry and a communications system that could hack into anything. In 1969, the Silence trapped Melody Pond in one, but she managed to break it open and escape. (TV: Day of the Moon)
Behind the scenes[]
In the real world, the term "exoskeleton" refers only to natural protective external skeletons of Earth animals, rather than technology to support life. Real-world exoskeletons do, however, fulfil much the same functions to the animal as their DWU counterparts do to their inhabitants.