A Chapter is a self-contained Space Marine army usually made up of a thousand or so Space Marines, as well as a large number of administrative and functionary personnel.
The contemporary – 40th millennium – Space Marines are divided into approximately 1,000 Chapters. Each individual Chapter is autonomous and constitutes a complete army with its own leadership, support and administrative staff, reliant only on its own Chapter members. Each Chapter has its own traditions, specialities, beliefs and practices. Each Chapter is led by a Chapter Master, considered to be the most powerful, wise and experienced warrior his Chapter possesses.
As well, the influence of each Chapter varies, depending on their specific holdings and responsiblities. For example, the Ultramarines rule over an entire star system from which to draw their resources and recruits, while Chapters like the Black Templars and Blood Ravens are based on their mighty Chapter fleets and roam the Imperium in a nomadic fashion, drawing aspirants from a variety of recruiting worlds. Others still, such as the Space Wolves, Blood Angels and Iron Snakes, are limited to a single world in which they recruit.
For the most part, Space Marine Chapters can be divided into two categories; Codex Chapters, namely those who follow the guidelines for organization set out by the Codex Astartes (to varying degrees, depending on the Chapter); and non-Codex Chapters, those who do not. For example, the Ultramarines are known for following the Codex with an almost religious devotion, given that the text was written by their founder and Primarch, Roboute Guilliman. Meanwhile, the Space Wolves reject the Codex utterly, organizing themselves in a far different fashion than most Space Marine Chapters. Finally, there are Chapters like the Blood Angels, who accept the organizational doctrines of the Codex for the most part, save for a few specific concessions to their own philosophies and Chapter cult.