The Nestene Consciousness is a telepathic gestalt entity of pure psychic energy, somewhat amorphous or cephalopod-like in appearance. It developed on the planet Polymos, which it completely infused, destroying all other life on it.
It has the ability to transport sections of its mind through radio waves and possess plastic, shunting some of its consciousness into it or create a body out of it, allowing it to manipulate its surroundings.
Consciousness bits travel through space in hollow spheres known as energy units. These units make planetfall in formation, appearing like normal meteorites . Once the consiousness within those spheres is recombined and placed in an environment tank the Nestene will take on a form that it believes to be perfectly adapted to the planet on which it has landed.
Usually, the first spheres to arrive will attempt to place a local being under some form of mental control to regroup all the units together and fashion plastic automatons known as Autons, which may include replicas of key authority figures as well as simpler automatons. The Nestene will then use the Autons as an army to conquer the planet.
Source: Doctor Who Universe.