Just wanted to get this doubt out of the way, specially since the Clone Wars game is coming out, and he will most likely be a character and will need a concept clasification.

IMO, this is a pretty tough call. I don't know if it's a robot with some human biological features implanted inside it (i.e. Dr. Gero aka Android 20) or a person with and extreme robotic makeover. Then again, their is another question, Grevious is also an alien from what I can tell, so can he even be catologued in the Android or Cyborg concepts?

I think Grievous would be best classified as a Cyborg because he still has some of his original body left, but is mostly machine at this point. 

I'm pretty sure he was originally a biological being, that just at one point got a robot body.

He definitely uses a lot of his old organs, shown by how his lungs got messed up enough to give him that cough, and how Obi-Wan killed him with a blaster shot to the gut. If you still rely on all of those, and aren't just a brain in a robot body, I'd classify that as a cyborg.

Android=Human looking robot
Cyborg=flesh and machine parts.