SPACE MARINES
The Space Marines are the product of genetic engineering, intensive training, biochemical alteration and implant surgery which takes them to the limits of Human development and beyond. A Space Marine can survive terrible wounds, heal with superhuman speed, and perform incredible feats of strength and endurance. A Marine is genetically related to all the other members of his Chapter, since each Marine carries some genetic material from the Chapter's founder, or Primarch - one of a small group of super-humans created by the Emperor and his scientists.
In numerical terms, the Space Marines are a small force compared to the countless billions of Imperial citizens, but their superhuman ability and fearsome reputation makes them worth many times their number in conventional troops. They are held in awe throughout the Imperium.
Each Space Marine Chapter is a fully self-sufficient fighting force, with its own space fleet and machine pools as well as support and technical personnel. The Techmarines are trained by the Adeptus Mechanicus, and can repair almost any piece of military equipment, from a bolter to a spaceship.
A Chapter's fleet is its home, providing accommodation, training facilities, machine shops, armories, shuttle silos, chapels, a mausoleum and every other facility the Chapter requires. The fleet travels endlessly through the galaxy; task forces split off from it for individual missions or campaigns, and rejoin the fleet when the mission is accomplished.
The Chapter provides for all a Marine's needs. Tradition and ritual are of great importance, with as much emphasis on moral welfare as on physical training and combat skills. The Chapter is a Marine's family; all other ties of blood and loyalty are renounced. A Space Marine is devoted to the Emperor, to his Chapter, and to the craft of war; these things are his entire life.
Each Chapter is headed by an Imperial Commander - during the Horus Heresy, most of the Commanders were Primarchs. In addition to their support and technical personnel, a Chapter contains tens of thousands of Marines (Note that after the Heresy new Chapters were formed with far smaller complements so that no Commanders would ever wield the same power as Horus).
Space Marines may be recognized by their distinctive powered armor - which includes air and blood purifiers, drug injectors and communications equipment. Neuroplastic fiber-bundles replicate and amplify the wearer's movements, so that powered armor is no more cumbersome to wear then a normal suit of clothes.
Each Chapter has its own colors and insignia, which are used on its armor and vehicles as well as on the flags and banners which are often carried into battle. It also has its own particular rituals and traditions. The Space Wolves, for instance, have a completely different Litany of Combat then the Ultramarines. Even the most basic activities, such as weapons checks and maintenance routines are turned into rituals; a Marine's every action must express his devotion to his Chapter.
In the earliest stages of the Horus Heresy, the Space Marines who sided with the Warmaster kept to the original colors and rituals of their Chapters. Later on, however - particularly after the death of Horus and the withdrawal from the Sol system - this began to change.
The litanies proclaiming devotion to the Emperor had already been dropped, and they were replaced with declarations of allegiance to Horus. As the feral world cults introduced by Horus spread throughout the Traitor Chapters, most Chapter rituals changed beyond recognition. Insignia and colors also began to change; most common was the replacement of imperial insignia with the Eye of Horus or with symbols from the feral cults. Some Traitor Marines went so far as to re-paint their armor in new colors.