The Space Marines are the most feared and respected of all of mankind's warriors. Standing over seven feet tall, genetically engineered then extensively trained they are the ultimate human fighting machine. Although relatively few in number, a Chapter comprises a thousand Space Marines, their prowess and bravery means that they can defeat their opponents who outnumber them many times over.
Equipped with the best arms and armour the Imperium has to offer, the Space Marine armies are also extremely mobile, with transport that allows them to move swiftly to deal with an enemy threat, or to strike suddenly to seize a vital strategic objective.
Space Marine tanks and other vehicles are well-armed and fast, allowing them to keep up with the swift moving infantry, while Space Marine land speeder and bike formations excel at lightning raids and flank attacks.
The Space Marines' dedication and fearless devotion to their Chapter make them ideal defensive troops, their heavy armour and excellent training allowing them to take on seemingly impossible odds only to emerge battered but victorious!
-from the Warhammer 40,000 rule book
Space Marines are one of the most potentially powerful armies, but they are tricky to play. They are always outnumbered, and therefore need to be careful. I've heard tales of gretchin taking down Terminators through sheer weight of numbers. If you play Space Marines, be sure to think of some strategy to counter-act being outnumbered!