Geonosis is an Outer Rim planet. In terms of physical size it is very large, however has quite low gravity. The surface of the planet has been made desolate due to constant solar radiation storms, and massive sandstorms. Furthermore there is very little surface water, which was another contributing factor for why there are very few living organisms on the surface. The architecture is large termite and ant like mounds made it easy to mistake the planet as being uninhabited, which made it a very good place for a separatist hideout.
The 2 largest Inhabitants of the planet are: The red Algae, this is one of the few organisms that can survive on the surface despite all the hardships. The other inhabitants are the more famous Geonosians who are more insectoid than humanoid, however still speak and understand Basic and are relatively intelligent, the Geonosians had been forced underground following a near mass extinction of their race after an immense sandstorm, this mass extinction happened again when a comet hit their largest moon causing debris in the form of asteroids to hit their planet and kill the weaker part of their population.
During this time, Geonosis was being run by Archduke Poggle the Lesser. He sided with the confederates and separatists. He built several droid foundries on his planet. 2 years later in 22 Battle of Yavin (BBY) The Archduke organised a meeting of the separatist leaders with Count Dooku on Geonosis. Unfortunately, Obi-Wan overheard their plans and informed the Jedi Order. Obi-Wan, Anakin and Padme were forced to fight in a gladiatorial arena against some large imported beasts: a Reek, a Nexus and an Acklay. Luckily it was then that Mace Windu and 200 other Jedi arrived and helped fight off the droids.
It was here that Count Dooku first fought in the films, and also the first time Yoda did. Jango Fett met his demise here at the hands of Mace Windu. The clone army was first revealed here. Furthermore the planet staged the first battle between the clones and the droids.