Space Marine Chapter
The Space Marine Chapter is the largest commonly-used organizational unit of Space Marines, nominally consisting of a thousand Marines, give or take. As of the 41st Millennium, there are approximately one thousand Space Marine Chapters operating in the Imperium of Man.
Each Chapter is descended, in one way or another, from the twenty original Space Marine Legions founded to wage the Great Crusade -- in fact, the term "Chapter" began as one of several names for a roughly thousand-strong division of Marines, as a thousand Marines was few enough to be logistically manageable while numerous enough to handle almost any situation, especially with the assistance of their Imperial Army regiments. However, some Legions preferred to operate at full strength all the time, especially the Sons of Horus (judging from the books, the Vengeful Spirit housed the Legion's whole first ten companies) and the Death Guard (operating most of the time as a single force of ~90k+ marines). It says something about the sheer manpower of these armies that that large portions of the Legion were often benched during a campaign.
The Horus Heresy proved that having a quarter of a million Marines and millions of Guardsmen under the command of one corruptible individual was too dangerous, so Roboute Guilliman (who also originally had a quarter of a million of Marines and millions of Guardsmen under his command), with the grudging assistance of the surviving loyalist Primarchs, instituted the Codex Astartes reforms, where the remaining loyalist Legions were divided into Chapters of no more than a thousand Marines. This event was known as the Second Founding, with the creation of the twenty original Legions being retroactively designated the First Founding.
The First and Second Foundings were unusual compared to the rest; in both cases, there were plenty of Space Marines to be had, so the founding of new Legions or Chapters was basically a re-organization. By the time of the 41st Millennium, Space Marines are made in much fewer numbers, so founding a new Chapter is not done lightly -- in fact, only twenty-four Foundings have been held since the Second.
First, there has to be sufficient gene-seed. If there isn't enough in storage, more will be made by implanting some into slaves, extracting the progenoids, using them to make two sets of gene-seed, and so on. In a century, one set of gene-seed can be cloned into over a thousand, which is more than enough for a new chapter. Once there is enough gene-seed, a suitable set of Aspirants will go through training to become the first battle-brothers, while a few Veterans from a chapter that already exist will become the first Master and Captains of the Chapter. This is a great honor for those Veterans chosen, and for their Chapter as well.
A single Marine is extremely powerful; a full Chapter is overkill for almost any situation save the most dire (though these situations are naturally the ones we hear about). To ensure that enough Marines are committed to a given battle (but no more, since they are too expensive to risk where they aren't needed), all Chapters have some organization scheme for dividing into smaller detachments. The majority of Chapters use the organization laid out in the Codex Astartes to some extent: a company of ten Veteran Squads, four Battle Companies with six Tactical Squads, two Assault Squads, and two Devastator Squads, four Reserve Companies (two with ten Tactical Squads, one with ten Assault Squads, and one with ten Devastator Squads), and a Scout Company.
In-game, one's choice of Chapter generally has no effect on gameplay -- of course different Chapters have different color schemes, and an army's lore may suggest taking certain units in greater or lesser quantities, but they all basically draw from the same army list. That said, some Chapter Masters and Captains, if chosen, can grant certain special rules to the army they lead, and some Chapters (Space Wolves, Black Templars and successors [prior to 6th edition], Blood Angels and successors, Dark Angels and successors, and Grey Knights) get their own Codices.
This changed with 6th Edition; now, your choice of chapter does affect what "Chapter Tactics" they get. For example, the Red Talons, an Iron Hands successor, has the Iron Hands' version of the Chapter Tactics rule, while Howling Griffons use the Ultramarines'. Some of the special snowflake chapters, like the Red Scorpions or Black Templars, get their own, but for the most part, it's based on whichever First Founding chapter they come from.
- Chapter Master (game), a game-in-progress in which you get to run your very own Space Marine Chapter (and probably get declared Excommunicate Traitoris by the Inquisition).
- Space Marine Chapter Creation Tables, roll-tables from the Deathwatch Rites of Battle supplement that let you randomly generate your very own Space Marine Chapter.