Hermethican

According to the Wire, she was sentenced to death by her people and eventually executed. However, she became a being of pure energy, and escaped via a mobile communications device, and fled across the universe to Earth in 1953 where she became trapped in a television set. (TV: The Idiot's Lantern)

Behind the scenes
The true form of the Wire or her species is unknown, since she was using the image of a BBC continuity announcer that was on the television when she possessed it.