Taken from the Wookieepedia

Dooku, a Force-sensitive human male, was a Sith Lord who led the Confederacy of Independent Systems as Head of State during the Clone Wars. Although he was once a Jedi Master, Dooku left the Jedi Order and the Galactic Republic after growing disillusioned with the corruption in the government. Turning from the light he once served as a Jedi, Dooku joined the dark side of the Force and became the apprentice of Darth Sidious, the Dark Lord of the Sith, taking on the name Darth Tyranus and reclaiming his title as the Count of Serenno. With his master, who was secretly Supreme Chancellor Sheev Palpatine, Dooku conspired to overthrow both the Republic and the Jedi so that the Sith could reclaim their former dominance. To this end, he oversaw a growing secessionist movement that ultimately resulted in the formation of the Confederacy of Independent Systems. In addition to rallying rebellious star systems to the Separatist cause, Dooku was involved in the creation of the clone troopers of the Grand Army of the Republic, having supplied the Kaminoans with the clone template, Jango Fett, as well as the inhibitor chip containing Clone Protocol 66. As a result of Dooku and Sidious's machinations, the galaxy became engulfed by the Clone Wars twenty-two years before the Battle of Yavin.

Count Dooku led the Confederacy's battle droid army throughout the war, turning star systems against the Republic through political maneuverings and military force. Though he was bound by the Sith Rule of Two, Dooku trained apprentices of his own, with the intention of one day succeeding his master. He first trained Asajj Ventress; later, after Sidious ordered him to dispose of Ventress, Dooku took on a second apprentice, Savage Opress. The tutelage was short-lived, as Opress had been used by Ventress as a tool of revenge against her former master. Toward the end of the war, Dooku took on a third apprentice, the fallen Jedi Quinlan Vos, with whom he plotted to overthrow his master. This apprenticeship, too, was cut short, as Vos was redeemed in the Force with the help of Asajj Ventress.
The final days of the war saw a battle over Coruscant. In a duel aboard the Separatist flagship Invisible Hand, Dooku was defeated by Jedi Knight Anakin Skywalker, who beheaded the helpless Dooku at Palpatine's command. Dooku was soon replaced as Palpatine's apprentice by Skywalker himself, who fell to the dark side and became Darth Vader. Despite Dooku's death, the plans he helped set into motion during the Clone Wars came to pass, with the destruction of the Jedi Order and the rise of the Galactic Empire.