They don't talk about were there from but basically they discovered time travel and go back and punish criminals through out history unpunished. They are normal size but they use a shrink ray so a group of them can fit in a robotic body.
The Teselecta is a shape-shifting robot that can disguise itself as anyone. It's used to punish criminals. It was used to investigate and attempt to punish Adolf Hitler in "Let's Kill Hitler" and later returned disguised as the deceased Father Gideon Vandelaur to investigate the Silence in "The Wedding of River Song".