In the Warhammer 40k universe, there were originally 20 chapters of space marines.
Two have been lost, leaving 18 today (meaning in the 41st millennium).
There's a convenient even split within these 18 legions, with 9 being loyalist troops in service to the fascist Emperor and the other 9 being dedicated to the selfishness forces of chaos.
Because each chapter of space marine has factions within them, you do sometimes hear a lot of names being thrown around, and it can make it hard to remember what's at the top of the family tree.
This is a list of the 9 space marine loyalist chapters and the 9 traitor legions as they exist in the 41st millennium.
They're listed by legion number, along with their primary colour scheme (using Contrast Paint name, when available from Citadel), a notable trait, and Primarch.
- Dark Angels: Loyalist, Dark Angels Green, chivalry, Lion'el Jonson
- <Redacted>
- Emperor's Children: Chaos, Syish Purple, Slaanesh, Fulgrim
- Iron Warriors: Chaos, Basilicanum Grey and Aggaros Dunes (silver and gold), methodical, Perturabo
- White Scars: Loyalist, Apothecary White and Blood Angels Red, fast strike forces, Jaghatai Khan
- Space Wolves: Loyalist, Space Wolves Grey (blue-grey), viking warriors, Leman Russ
- Imperial Fists: Loyalist, Iyanden Yellow, heavy artillery, Rogal Dorn
- Night Lords: Chaos, Leviadon BlueAggaros Dunes (blue and brass), sadism, Konrad Curze
- Blood Angels: Loyalist, Blood Angels Red, artistic blood lust, Sanguinius
- Iron Hands: Loyalist, Black Templar (black and grey), augmetics, Ferrus Manus
- <Redacted>
- World Eaters: Chaos, Flesh Tearers Red and Aggaros Dunes (red and brass), Khorne, Angron
- Ultramarines: Loyalist, Ultramarines Blue, space marines, Roboute Guilliman
- Death Guard: Chaos, Militarum Green, Nurgle, Mortarian
- Thousand Sons: Chaos, Akhelian Green and Iyanden Yellow (azure and yellow), Tzeentch and psykers, Magnus the Red
- Black Legion: Chaos, Black Templar and Aggaros Dunes (black and gold), use but do not worship chaos, Horus
- Word Bearers: Chaos, red and silver, first heretics and worships daemons, Lorgar
- Salamanders: Loyalist, Warp Lightning (Green), blacksmiths, Vulkan
- Raven Guard: Loyalist, Black Templar and Apothecary White, stealth, Corvus Corax
- Alpha Legion: Chaos, blue, spy and subterfuge, Alpharius and Omegon
Lots of material to explore!
You really get to know the Primarchs in the Horus Heresy books, and obviously the Black Library and Codexes have lots of material to read.