Originally Posted by StarWars Databank
A twisted melding of flesh and metal, General Grievous' body is a deadly weapon forged by the cutting edge developers of the Confederacy. Grievous' living matter was encased within his precision-engineered artificial body; inside the hardened carapace beat the heart of a remorseless killer. A pressurized gut-sack held his vital organs, while his skull-like mask contained his living eyes and brain. Making the horrific amalgam more unpleasant was a persistent wet, hacking cough coming from his ravaged lungs.
From the OfficialŽ Site
Be sure to read the EU
section as well, it contains more on Grievous' history...