From The Official TF2 Wiki:
"The Minigun, known affectionately as "Sasha", is the default primary weapon for the Heavy. It is an enormous Gatling-style machine gun with a large rotating barrel, complete with a large white underslung ammunition case and two handles."
The Minigun has a black barrel and grip with a dark silver bullet holdier. It deals 4-12 damage at long range, 12-30 damage at medium range and 32+ damage at close range.
The TF2 Minigun is very similar to the TFC incarnation, but it has one difference; you can wind up the gun without firing by holding down the alt fire key. This is very useful for when you're defending and the point you're defending is frequently attacked. Instead of having to wind up the Minigun every time you spot an enemy, you can just hold down the alt fire key and mow down any incoming enemies without having to wind down when there's a lull in the fighting.