Count Dooku a.k.a Darth Tyranus (from the word "tyrant") was a powerful Sith lord apprenticed to Darth Sidious. He was originally the Jedi apprentice of Master Yoda but was, in his later years drawn to the power of the dark side. He was also the master of Qui-Gon Jinn.
Although, he was first tempted by the Dark side as a Padawan, when his best friend (Lorian Nod) became a Sith Lord, Dooku rose to be a mighty Jedi Master. In later years, he even lead the Jedi against the Mandalorians; the war against Boba Fett, where half the universe's Jedi were killed. However, shortly after this battle, the weary Dooku lost faith in the slowly corrupting Galactic Senate and the Jedi for serving it. He was greatly angered by the unwillingness of the Jedi Council to aid oppressed worlds and systems. The death of his apprentice and friend, Qui-Gon Jinn was the final blow to his already waning faith.
It is at that time he became apprenticed to Darth Sidious. His true dark nature began to emerge as he became colder and crueler; often viewing others as mere objects to further his ends. He further expressed extreme hatred for non-humans, seeking to place man above all, as the supreme, universal Authority. He then became the leader of the Confederacy of Independent Systems, the separatist group that was rebelling against the Galactic empire. He was behind several intrigues, especially involving the erasure of logs in the Jedi archives.
He was most renowned, however, for his involvement in the Clone Wars. During the war, Count Dooku, trained several non-Sith apprentices eg: General Grievous and also tempted, imprisoned and sentenced Obi-Wan-Kenobi to death but was thwarted. He battled quite a number of Jedi too, most notably, Yoda (to a draw) and even cut off the right forearm off of Anakin Skywalker during the Clone Wars. He also procured and passed on the first designs of the Death Star to Darth Sidious.
Close to the end of the Clone Wars, he and General Grievous "captured" Palpatine, recently elected Chancellor, to deceive the corrupt Senate into believing Palpatine had been kidnapped, thus creating the necessary sympathy to hand over even more emergency powers to the him. However, Anakin Skywalker and Obi-Wan Kenobi came to rescue the Chancellor/Sith lord. Count Dooku was secretly ordered to dispose of Obi-Wan but to spare and surrender to Anakin. This he did (although Kenobi survived). In the fight against Anakin, however, he found himself battling for his life and eventually Anakin Skywalker cut off both Dooku's hands.
Darth Sidious had promised to intervene if Skywalker defeated him. Count Dooku was betrayed. The words "Do it" came from the Chancellor in the voice of Darth Sidious and Dooku, in his last moments realized he was a simple stepping stone for Sidious' true intended apprentice: Anakin Skywalker. Then, weak and defenseless, Count Dooku was decapitated by Anakin's lightsabers.
Personality and traits
Dooku was widely known for his calm and strategic moods. He uses this to his advantage in battle when his opponents see his calm emotionless face it intimidates them and make them fight sloppier.
When Dooku became sith he was very loyal to his master Darth Sidious, Unlike his trainees General Grievous and Asajj Ventress who frequently disobey and put their own gain in front of their master, Dooku obeyed every order Sidious told him without question and eventually led to his death.
Powers and Abilities
Count Dooku was one of the greatest Jedi/Sith in the Galaxy. Although he was a bit of a demagogue and delved deeply in philosophy, he was extremely knowledgeable in the ways of the force, frequently using it in battle. As a Sith, he possesed a red lightsaber with a curved handle. With it, he was a deadly duelist.
In fact, he was one of the few force-users known for using the second lightsaber form, Makashi, as it was obsolete by his time, or at least was considered so. He was thought of as a match for Yoda and was one of only two (the other was Yoda) beings capable of defeating Mace Windu in combat.
Behind the scenes
Count Dooku was played by British actor Christopher Lee in "Star Wars Attack of the clones" and "Star Wars Revenge of the Sith".