The Watchers were among the first planetbound intelligent humanoid species in the universe. The Watchers are led by a Watcher known as the One, who is physically far, far larger than the other Watchers and who serves as a repository for all data and records into which the other Watchers upload all the information they collect,.
They started with the belief that their knowledge belonged to the universe, and so decided to share it with other younger species. They came to the planet Prosilicus, interfering with their evolutionary and technological development. Despite the well-intentioned actions of the Watchers, the species self-destructed in nuclear warfare. Horrified, the Watchers swore a vow of non-interference they’ve maintained for millions of years and, leaving their homeworld, dispersed throughout the cosmos for observation purposes.
The Watchers possess the ability to manipulate reality at will, at least on a local level, can teleport themselves almost anywhere in the Universe instantaneously. They appear to be able to either simultaneously watch multiple Universes and Dimensions or to be in more than one place at the same time, and possess vast mental, physical, and energy-manipulating powers. They can also alter their shape and increase and decrease their size at will. There appear to be no secondary sex differences between male and female Watchers. Even if rarely, Watchers still reproduce sexually, however, and rudimentary family relationships still provide care and training for their young.
The Celestials are of comparable age and power to the Watchers. But their agenda of genetic engineering of younger species is in direct opposition to the Watcher agenda of non-interference. The two species have thus been engaged in conflict for billions of years, a conflict that culminated fairly recently in1995 and was not resolved one way or the other.
Source : Marvel Universe « Fantastic Four #13 (April 1963) »