A hand was an organ on many humanoid species, the primary function of which was grasping, holding, typing, etc. The hand played an integral part in the sense of touch, since many humanoid species used their hands for tactile feedback.
Humanoid hands tended to vary with both number of hands and number of digits on each hand: Voth and a species from Tau Alpha C had three fingers per hand, while Humans had five; as well, Edosians had three hands, while Terellians, and a pianist on Qualor II, named Amarie, had four hands.
In 2151, Doctor Phlox treated Crewman Alex, who had first-degree burns on his right hand, from the seal of a nitrogen valve having blown when he had been trying to reroute the valve. The hand injury (and a similar burn on Alex's forehead) was treated with dermaline gel. (ENT: "Dear Doctor")