Char is basically just a big ball of rock and lava, not unlike Mustafar. It is covered by an atmosphere of Oxygen and Sulphur, laced with a layer of ash. This makes it perfect for creepy races like the Zerg to make their home.
Char is described by non-Zerg as 'hell in the mortal realm'. Ash clouds block light from the binary stars which it orbits, magma seeps to the surface, and rock formations collapse and reform every few minutes.
Zergies, possessing some intelligence, decided to live underground. They had (and still probably have) a massive network of tunnels running beneath the surface to allow them comfortable travel.
The planet was invaded by a Renegade Human army during the Three-Way War, and the trillions of Zerg on the surface were reduced to millions through the use of some super-overpowered device that no-one knows the name of (Maybe related to the Composer). After the Zerg were accepted into The League, they were allowed to live on Low Charity and other Flood populated worlds, so Char was basically abandoned. Many Zerg still live below the surface, eeking out a living by scavenging.