The Tau are a young xenos race that have their own empire in the eastern fringe of the galaxy. Despite being only 5000 years old, they have technologically surpassed the Imperium of Man in many areas. Despite this, as they have very weak Warp signatures, they possess no psykers, and thus cannot make use of faster-than-light travel. However, this means they are also less susceptible to daemonic possession (though they are not immune). The Tau follow an ideology called the Greater Good, and will recruit many other races to fight on their behalf to spread their ideology.
The Tau were discovered five thousand years ago by Adeptus Mechanicus explorers. At the time, they were a primitive stone-age race, with each caste of Earth, Fire, Water, and Air, at war with one another. Shortly after the discovery, a warp storm isolated the planet, and didn't abate until a few centuries ago. In the intervening time, a fifth caste called the Ethereals taught the other castes of the Greater Good, and united the world into a technologically advanced empire. The Tau welcome other xenos races and even humans (whom they call Gue'la) into their empire. For example, the Kroot are commonly recruited as mercenaries, while the Nagi serve as “advisors” to the Ethereals.
When the Imperium discovered their existence, they immediately called for the Tau's extermination. Though s minor threat in the grand scheme of things, the Tau could not be allowed to convert any more Imperial worlds. Despite its small size and lack of experience in galactic warfare, the Tau's technology was advanced enough to stall the Imperium in an intractable stalemate. Only the appearance of the Tyranids in the Eastern fringe forced the Imperium to withdraw.
While the Tau still hold faith in the Greater Good, their first taste of the outside world has dimmed their enthusiasm. Their leaders have only just discovered the ruinous powers, and many other races prove to not be interested in peace.