Autons are, simply put, human-sized dummies made of plastic, although they are not the only creations of the Nestene Consciousness they are one of the most recurring and recognized.
They first appeared in the episode "Spearhead from Space" where they are used by the Nestene Consciousness to wipe out Earth's population so they can colonize Earth.
They returned in "Terror of the Autons", where they fought against the Third Doctor together with the Master.
At the first appearance of the Ninth Doctor, they reappeared along with the Nestene Consciousness and tried to conquer Earth, but were deactivated when the Doctor's companion Rose Tyler destroyed the Nestene.
When the Daleks, the Cybermen, the Judoon, the Sontarans and the Doctor's other enemies wanted to trap the Eleventh Doctor in the Pandorica to save the universe, more realistic Autons disguised as Romans (which included a version of Rory Williams) were created by the Nestene to perform a scenario that would lure the Doctor to the Pandorica.