There are actually different armies within this one. When the Space Marines were created, they were put into legions. These legions were broken up into the chapters that exist today. Each Space Marine chapter has its own colors, organization, and background. A chapter is normally broken into companies, which are broken into squads. A chapter usually has about 1000 men, forming 10 companies of 100 men each.

The Space Marines are the defenders of the Imperium. They were created in the likeness of the Emperor. They are super humans in the very sense of the words. A person is taken at the age of 12 or 13 to become a space marine. They are taken to the home planet of the chapter that recruited them. Here, the metamorphosis from underworld scum to the finest worriers in the universe begins. The first things to be implanted into the adolescent are two gene seeds. These adapt the body of the boy to be able to accept all of the other implants. The gene seeds are recovered from a marine when he falls in battle, so they can be used again. The new recruits are then given a series of other implants, and the result of all of these is a man who is 8ft tall. He has huge muscles and bones, his ribs have formed one solid plate of bone. His wounds clot almost instantly. He can survive in extreme cold or heat, or even a limited vacuum, and he can breath under water for a while. His senses are all super enhanced. He can eat any poison, and survive any toxin in his blood. He can turn off half his brain so he can rest while staying aware of his surroundings. He can even absorb knowledge from his victims by eating them. He can secrete an acid in his mouth, and could eventually go through metal. And, the most extreme of all, under dire circumstances, he can enter a state of suspended animation, until he is found by his brother Space Marines. Now take this man, who could easily crush your skull in one hand, and put him in a huge suit of armor. This suit is fastened directly to the marines spine. When he moves, the powered armor moves too. It amplifies his already great strength and senses. It also has built in targeters and a radio, as well as lots of other technological devices. A marine is completely sealed from the elements in his armor.

Now that you know how insanely powerful Space Marines are, I will tell you about the army. The HQ section of the Space Marines is made up of the Captain, and his bodyguard. The bodyguard consists of 5 marines. One of these can become an apothecary, (a medic,) and one can become a techmarine, (who repairs weapons and vehicles). Also, you can make one a standard bearer, and one a veteran sergeant.

The elites for the space marine army are veteran Space Marine squads, and terminator squads. The terminators are veteran Space Marines that are lucky enough to get one of the rare suits of tactical dreadnought, (aka terminator,) armor. Terminator armor is very thick and very powerful. There are very few suits of terminator armor left, because the technology for making for them was lost. Normally, only the veteran Space Marines of the first company of a space marine chapter get this privilege. One other elite unit is the dreadnought. This is a Space Marine who was wounded and put in this machine. They are very powerful units.

For troops, the Space Marines have normal marine squads, called tactical squads, and scout squads. Tactical squads have 10 men, with one being thesergeant. It's called a tactical squad because one man can take a heavy weapon, and one can take a special weapon. All the other men are armed with the standard bolters. This way, they can face almost anything they come up against. The scouts have light armor and weapons, and are not as strong or well trained as their brothers, but they can infiltrate ahead of the main army, and disrupt the enemy.

For fast attack troops, the Space Marines have assault squads, as well as bike and land speeder squadrons. Assault marines wear jet packs, and can move across the field quickly to engage the enemy with their close combat weapons. Bike squadrons also move quickly, and with the support of an attack bike, they can hit heavy targets also. Land speeder squadrons can move very fast, fly over terrain, and can mount a variety of heavy weapons. They can quickly flank the enemy an tear them apart.

For heavy support, the Space Marines have devastator squads, as well as tanks. A devastator squad can arm four of its ten members with heavy weapons, so they can take down very heavy targets. The space marines also have a few tanks. The rhino is a transport tank that carries 10 men. All other space marine tanks, except for the land raider, are modifications of the rhino chassis. The razorback replaces some of its cargo room for the weapon turret, but it can still carry 5 men. The predator has multiple heavy weapons, and no cargo room. The vindicator has a huge cannon on the front. The land raider is a lot bigger than the rhino based tanks. It can carry 10 men, and it also has lots of weapons and armor.

There are also a bunch of special characters for the space marines. These are really powerful models, and both opponents have to agree to their use.