The Confederacy of Independent Systems, CIS, or Separatists was a breakaway faction of the Galactic Republic. They were led by Darth Sidious (Palpatine), Darth Tyrannus (Count Dooku), General Grievous, and formerly, Assaj Ventress.
The CIS was formed after Count Dooku left the Jedi Order in 32 BBY, he met up with Nute Gunray of the Trade Federation and started a blockade on Naboo that led to a battle. After the battle ended Nute Gunray and Dooku went to Geonosis and met Archduke Poggle the Lesser. Later in 22 BBY, when the Clone Wars sparked, Count Dooku was found to be the leader of the CIS.