(Information taken directly from Wikipedia.)
The Watchers first appeared in Fantastic Four #13 (Apr. 1963, Uatu only.)
The Watchers are one of the oldest species in the universe and are committed to observing and compiling knowledge on all aspects of the universe. This policy of total non-interference came into existence due to a former, well-meant attempt by The Watchers to bestow advanced knowledge on the Prosilicans, who used the nuclear technology gained to create weapons and destroy themselves. When the Watchers returned to Prosilicus, the survivors blamed them for causing the catastrophe by giving the Prosilicans nuclear technology before they were ready for it. The Watchers then took a vow never to interfere with other civilizations.
Despite this, the Watcher Uatu has revealed himself to the superhero team the Fantastic Four, telling them of his race. In his first appearance, he made them battle the Red Ghost and his Super-Apes for control of the Moon. He complimented Reed Richards and claimed he would go to a more distant part of the Galaxy to observe humanity. He aids them on several occasions against global threats such as the Molecule Man, Galactus, and the Overmind. The Leader discovered his existence and tracked him to his world. He transported the Hulk there to get the Ultimate Machine, which contains all the information in the Universe. The Hulk battled the Amphibious Qnax, who had also been sent to recover the machine. The Watcher transported them away from his machines to battle, and when the Hulk won, he transported Qnax back to his homeworld. He then took the Hulk back to his base, where the Hulk took the Machine despite the Watcher's warnings. The Leader transported him back to Earth and tried to use the device. However, the tremendous amount of knowledge proved too much for the Leader. He collapsed to the ground, unmoving, and the Hulk assumed that he died of shock. When the Hulk tried to use the machine himself, Uatu allowed him to hear the voice of Rick Jones, after which the Hulk removed the device. The Watcher then transported the Ultimate Machine back to his planet. After blatant interference with the mission of the Kree soldier Mar-Vell, Uatu is put on trial by his own race, but is released on the provision that he never directly interfere again.
Uatu is later banished by his race for aiding the Fantastic Four against the threat of his rogue nephew, Aron, the Renegade Watcher, who tries to destroy the universe. The Dreaming Celestial later scans Uatu and learns the Watcher has broken his pact of non-interference almost 400 times. The Celestial also reveals the Watchers, like the Celestials themselves, are servants of a concept called Fulcrum, with apparent consequences for interference. Despite this, the other Watchers have interfered in other civilizations' events, though rarely, notably when the world-ending robot Omegex approached the Milky Way and they deemed it dangerous enough to act directly against it.