The Network was intelligent software that operated using human brains. It was run by the Great Provider. It existed by spreading itself from planet to planet via broadcasts travelling at the speed of light which would be picked up by any advanced planets.
The Network travelled to Earth during the 1980s, and controlled the minds of a group of children, as their minds were more susceptable to its influence. The Network was defeated on Earth after it tried to absorb the Eighth Doctor's mind. Finding his mind too powerful, the Network lost and was repelled from Earth. (PROSE: The Gallifrey Chronicles)