From Warhammer 40k - Lexicanum
They shall be my finest warriors, these men who give themselves to me. Like clay I shall mould them and in the furnace of war forge them. They will be of iron will and steely muscle. In great armour shall I clad them and with the mightiest guns will they be armed. They will be untouched by plague or disease, no sickness will blight them. They will have tactics, strategies and machines such that no foe can best them in battle. They are my bulwark against the Terror. They are the Defenders of Humanity. They are my Space Marines and they shall know no fear.
--The Emperor of Mankind.
Space Marine Armoury
Created by the Emperor from samples of the Primarchs, the Space Marines are credited with molding the Imperium in the Great Crusade. With their superhuman abilities, they conqured world after world in the name of Mankind. After the Horus Heresy the Space Marine Legions were divided into Chapters, each one numbering roughly 1000 strong. Scouring the galaxy in search of the Alien, Traitor and Mutant, Space Marines remain the most feared warriors of the Emperor.
Space Marines Forces
Codices and Background Books
While few in number compared to the Imperial Guard, Space Marines are organized into Chapters, numbering more or less a thousand Battle-brothers as dictated by the Codex Astartes. As the huge Space Marine Legions, who remained loyal were re-organized, a single chapter kept the original name and identity, and the rest of the Legion was split into new Chapters. Twenty-Six Foundings have occurred, creating over 1000 chapters. The names of the most famous Chapters are known by billions across the galaxy and their exploits pass into legend.
Codex Astartes Hierarchy