Featured in the episode “The Beast Below,” the Smilers are androids, mounted in booths all across the space faring version of Britain that left Earth in the 29th Century. They let people know what’s good and what’s bad. The terrified population keep them clean. Their heads can rotate to show three faces: happy, warning, or very dangerous indeed.
And if things get very dangerous, the Smilers can leave their booths and move around. They’re actually androids, an extension of the "starship’s" security system.
There are also the Winders, half-Smiler and half-human, who’ve taken an oath to serve the system. And all together, these unhappy people and automatons are doing a terrible thing, a fate from which only the Doctor and Amy can save them.