Uatu is a Watcher, a race of beings that watch. They are like recorders of the universe in that they are put places to watch over worlds and their peoples. They also had to take an oath not to interfere, which tends to be a problem with Uatu, as he likes earthlings to much to sit by and watch bad things happen. The Watchers are seemingly immortal (more on that later) and occasionally would come together to share the information they had seen with one another.
Uatu lived on the Blue Area of the moon, a part that has oxygen, in an abandoned Kree settlement. It was there that the Fantastic Four found him. He assisted the four on many occasions, even going as far as to lead the Human Torch in getting the Ultimate Nullifier, to thwart off Galactus. After several such occasions, Uatu was put on trial by his own people, but subsequently released after promising not to interfere again.
He was had a nephew, Aron, who went rogue and tried to manipulate the Fantastic Four into fighting the Inhumans as well as tried to end the universe completely so that he could start a new one. Luckily Uatu helped to stop him, but in doing so, was punished and stripped of his title.
More recently, Uatu appears in comics during times of upheaval or great change. He is usually featured once or twice during every big event, but more often that not, will simply stand there watching, or make only a small comment. He has also been the narrator of many What If?s as he can see into alternate universes.
However, it appears now, that in Original Sin, the next big event from Marvel, Uatu has been killed. I don’t know how, but his eyes were scooped out from his head. This means that whoever killed him has possession of all the secrets that he had accumulated over the years, secrets that no doubt will shock the MU.