They were parasites who lived by absorbing the neural energy of other beings. They were able to inhabit the bodies of physical beings and have technology that can render a corporeal being non-corporeal, leaving their body essentially brain dead.
In 2371, Chakotay and Tuvok of the USS Voyager encountered the Komar in their home nebula. Chakotay was separated from his body and Tuvok was possessed. He returned to Voyager and attempted to trick Captain Janeway into taking the ship to the Komar nebula. Fortunately, the disembodied Chakotay was able to help the crew uncover and defeat the impostor. (VOY: "Cathexis")