Grievous was a Kaleesh (or something). IIRC, his shuttle suffered a major crash, pulverizing most of his body. The Separatists took him, salvaged his brain and eyes, and developed a Droid body for him. He can't feel the force, or use it.
He's also the most kickass lightsaber wielder you'll ever find. He uses 4 Lightsabers, (trophies, I believe, from when he took down a team of Jedi on Hypori, featured in the Clone Wars, Season 1). His arms can split in half, giving him a total of 4 arms, and he can even use his feet to grasp and wield lightsabers.
He's the Supreme Commander of the Droid Army, as well.