Treacherous Space Marines who turned away from The Emperors guidance and embraced the foul Chaos Gods.
The first Space Marines to turn were The Word Bearers Legion Astartes. Their Primarch, Lorgar, used his relationship with his brother Primarch, the Warmaster Horus, to slowly corrupt him and his legion, The Sons of Horus, and Horus became the Arch-Traitor and leader of the Chaos Legions. Soon, The Death Guard, World Eaters, Alpha Legion, Emperor's Children and Thousand Sons legions joined Horus on his Dark Crusade to remake the Galaxy for Chaos and to destroy their father, The Emperor of Mankind. This civil war, known afterwards as The Horus Heresy, ended with the destruction of Horus, the death of The Blood Angels Primarch, Sanguinius (which caused the gene-curse in his legion), the splitting up of the remaining loyal Legions Astartes into smaller groups known as Chapters and, ultimately, The Emperors ascention to the Golden Throne due to the terrible injuries sustained in the fight against Horus.
Chaos Space Marines are a twisted and perverse mirror image of their Loyalist brethren, willing to commit foul and debased acts in the name of their Dark Gods. Chaos champions can be marked by their Patron God, making them more leathal and bloodthirsty in battle and, often, twisting their physical form away from the human template into something other. Chaos Sorcerors were once psykers, but now they are so corrupted by the powers of The Warp that their abilities are more akin to magic than psy abilities, as well as being able to summon Chaos Demons to help their fight against order and purity. Even their vehicals and buildings are corrupted by the touch of Chaos, becoming disgusting and foul to look upon. They leave a trail of destruction and desecration in their wake and the danger of corrupting any who come into contact with them and survive to tell the tale.