No, there aren't many types of Miniguns. There are many types of revolving battery guns (RBGs.) Calling all RBGs Miniguns is like calling all handguns Desert Eagles. The most common though is the General Electric/Dillon Aeromotive M134, 6-barreled RBG, caliber 7.62x51mm NATO, aka "Minigun." But I digress, most people do refer to the whole catagory as miniguns.
To answer the original question, there are two large sub-units mounted on the exterior of the M134 reciever (if you can call it that.) One is the feed delinking system that strips the ammunition from the desintigrating feed belt and loads it into the gun at a very high speed. The other is the (I think 28V DC) drive motor.
1 - drive motor
2 - feed delinker
Also, the one shown in the picture is a modern Dillon Aero one. The way those are mounted the gun is turned on it's side, so they have the motor and delinker on the left side, as shown. An original style mount has the gun turned with the motor and delinker on the bottom.
have too much time on my hands.
Oh yea, for more info and stuff on that and other models, check out www.montysminiguns.com