From Exterminatus Manual
Tactical Dreadnought (Terminator) armour is a rare and precious commodity in the Imperium, only an elite few of the emperors finest get to use it. All space marine terminator armour is used exclusively by the first company of each chapter. Originally designed for tunnel fighting terminator armour uses thicker and more durable materials than the standard power armour. To compensate for it's heavier weight the power lifting capacity of the suit is also higher then the standard power armour, allowing the wearer to carry weapons that would normally be too cumbersome for even a space marine.
The main battle weapon of Terminator marines is the a deadly combination of StormBolter and PowerFist, allowing to bring death at distance and in close quarters.
Better armour improves the terminators survivability considerable compared to regular marines, even if a Lictor gets the jump on you there is time to fight back. The powerfist packs a mighty punch, sufficient to give even a Carnifex second thoughts about getting into your melee range. Best used as a front rank offensive player, to tie up the enemy in close combat to give the rest of the squad behind time to shoot the enemy down. You need to keep a very close eye on your ammo usage as the StormBolter fires two bolts at a time. Extra bolts can be purchased from an Ammo Shrine.