From Exterminatus Manual
Heavy Weapons Marine (Heavy)
A heavy weapons operative makes up the final component of most tactical squads. Using the large, heavy, awesomely powerful weapons to support his squad the 'Heavy' class is the pinnacle of the imperial classes, able to blast his way though hoards of foes with relative ease. All that fire power comes at a price, the mass of his weapon reduces heavy weapons marines to a plodding pace.
Heavies can be recognised by their extended backpacks, weighted boots and optical targeters, not to mention their massive guns.
The Heavy Bolter with it's high rate of fire, large magazine and stopping power
The Missile Launcher, with it's large area of effect and very high damage missiles
The one small downside of all this awesomeness is the extra mass of this chunky gun and it's ammunition tends to make Heavy marines rather slower than their squad mates, so don't rely on being able to sprint out of trouble. This makes heavy marines choice targets for enemy sappers due to their high cost relative to their health.