Duplicitous former Jedi, turned to the Dark Side by Darth Sidious.
Count Dooku appears in two of the Star Wars prequel films, played by Christopher Lee, and was the political leader of the CIS during the Clone Wars. He started his path as a Jedi but fell to the dark side and helped start the Clone Wars and was used by Palpatine as a step on the path for Anakin Skywalker to become Darth Vader.
Dooku was born on the planet Serenno into a wealthy and noble family entitling him to the name Count Dooku. He was taken away at a young age by the Jedi in order to begin his training. Dooku trained under Thame Cerulian and eventually had Qui-Gon Jinn as his own apprentice. Count Dooku was highly respected as a Jedi and a swordsmen and it was thought that only Jedi Masters Yoda and Mace Windu could match him.
Count Dooku spent 70 years with the Jedi Order until a horrible battle left his belief in the Jedi shaken. Soon Senator Palpatine, then representative from Naboo, held a powerful influence over the Count and convinced him to leave the Jedi Order and turned to the dark side of the force. Dooku replaced Palpatine's slain apprentice Darth Maul and the title Darth Tyranus was given to him.
Once the Clone Wars began, Dooku became the political leader of the Confederacy of Independent Systems. He recruited powerful allies including his apprentice Asajj Ventress and the Jedi-killing General Grievous. After years of leading the Separatists and drawing more and more systems to his side Palpatine planned to soon bring the war to an end. It was at the battle over Coruscant after helping lead the attack to kidnap Supreme Chancellor Palpatine that Dooku took on Jedi Knights Obi-Wan Kenobi and Anakin Skywalker easily dispatching with the two. Dooku's taunting encouraged Anakin to unleash his anger and hatred and turn the tables on Dooku, disarming him and having two lightsaber's to the Sith's throat. Despite some resistance, Anakin was convinced by Palpatine to kill Dooku instead of arresting him and unknown to Anakin his actions began him down the path of the dark side of the force.
Despite his fall to the dark side Dooku was known as an apt politician and speaker both as a servant of the Jedi and of the Separatists. Being very powerful in the force he was one of only a handful of Jedi in their history to voluntarily leave their ranks.