Geonosis is a Mars like planet where the inhabitants are Geonosians, anInsectoid like alien lifeform. The planet is where the Separatists meet to discuss their next plan of action. Its not pure rock, there are sand dunes. It also has a ring much like some of our outer planets in our Solar System -> Saturn and Uranus. It should be noted that all the gas giants in the solar system have rings but that the rings on Jupiter and Neptune are very faint. Obi Wan Kenobi follows the Jango Fett to the planet. Anakin Skywalker and Padme Amidala respond to Obi-Wan Kenobi request for help before a full Jedi task force led by Mace Windu arrives. In terms of films, it only appears in Star Wars - The Clone Wars and provides the back drop to the climax.
Obi-wan Kenobi fellows Jango Fett to the planet and comes faces to face with the leaders of the Separatist organisation such as Count Dooku. On the planet, he is unable to communicate back to the Jedi Leadership and instead relays a message to them via Anakin Skywalker and Padme Amidala who against Obi-Wans wishes has gone to Tatooine.
When Anakin and Padme receive the message, they journey to the rocky planet in order to help Obi-Wan Kenobi however things do go to plan. Their attempts to rescue Obi-wan Kenobi leads them to a giant droid army factory where they avoid the machinery to survive. During the incident, C-3PO`s head gets knocked off and is replaced with a droid head. The correct helmet is latter corrected by R2-D2 during the Jedi vs. Separatist battle in the Arena.
It is on this planet that Anakin Skywalker looses his arm like what his son Luke Skywalker does on Bespin. Anakin`s hand is chopped off during a fight against Count Dooku. Count Dooku escapes before being defeated by Yoda.
The primary and most intelligent lifeform on the planet are the Geonosians, an insectoid lifeform that have wings to fly. In World of Warcraft, there is a race of insectoids called the Klaxxi on Pandarian that are influenced by the look of the Geonosians.