Space Marines
The Space Marines are the only playable army in the Warhammer 40k universe. They are genetically engineered supersoldiers clad in powered armour, and are generally regarded as the toughest bastards to ever server the Emprah. The average Space Marine is around eight feet tall, has bones that can repel present-day small arms fire, can breathe poisonous gas with no significant problems, is capable of spitting acid, and lives for hundreds of years. They are vastly more powerful in their fluff than they actually are in the Warhammer 40k games.
The Space Marines themselves were created by the Emperor using genetic information from the Primarchs, who were also created by the Emperor but subsequently kidnapped by the ruinous powers of Chaos and scattered across the galaxy while still children. All but two of the original twenty Primarchs were later recovered and re-united with the Space Marine chapters their genetic material had founded.
Space Marines are generally regarded as something of a "noob army", mainly because every twelve-year-old 40k player has a raging hardon for them, and almost every unit in the listings has at least a 3+ armour save, making them rather hard to kill. The Ultramarines in particular are an extremely popular choice of Space Marine chapter, and their blue design coupled with the small size of the miniatures often leads to them being referred to as "SMurfs". Thanks to Indrick Boreale, the Space Marines in general are frequently called "SPESS MEHREENS", or variations to that effect.
Anyone who doesn't play Space Marines is either a Tyranid, a faggot, or more than twelve years old. Probably all three, since no one cares about armies other than the aforementioned ones, and 'nids are totally fabulous. Also, twelve year olds play WARMACHINE as hard as they can.
The Space Marines of today look very different from the glory days of Rogue Trader, where they earned the nickname beakie for their fuckawesome helmets. GRIMDARK fluff was less prevalent, and Mahreens were psychotic killing machines, not monks for TEH EMPRAH.
/tg/ Space Marine Chapters
One of /tg/'s favourite pastimes is creating new and exciting chapters of the Adeptus Astartes based on silly concepts. /tg/'s homebrew chapters include: