For questions about Space Marines in the W40k universe. Always use in conjunction with the [warhammer40k] tag and any other relevant tags.
Space Marines are genetically enhanced warriors in the Warhammer 40000 universe. They were created of the genetic stock of Primarchs by the Emperor of Mankind.
In the 40th millenium, there are around 1000 chapters of different strength. Some follow the in the Codex Astartes fixed maximum of 1000 Space Marines per chapter rigourosly, some ignore the Codex completely.
Space Marines are separated in Loyalist, i.e. loyal to the empire and emperor of mankind, and Chaos Space Marines. Chaos Space Marines follow the Dark Gods and are mostly split in Chaos Warbands.