Chaos refers to the universal and usually malign spiritual force embodied by the malevolent intelligent entities comprised of psychic energy that live in the Warp that is the equal and opposite of Order. These entities are mostly daemons, but the term also encompasses those mortals who have thrown in their lot with Chaos. Chaos is also itself the turbulent psychic energy that comprises the Immaterium, for Chaos is the limitless ocean of spiritual, psychic and emotional energy that defines the Immaterium and underlies the 4-dimensional material universe of space-time. It is a great and raw force of change and power, and is both physically and spiritually corrupting, though it is not in itself necessarily "evil." While its adherents and servants act in ways that are often malevolent, some devotees of Chaos are more dedicated to the service of freedom and change than of the pure selfishness that Mankind and most other intelligent species define as nefarious.
With all the obvious dangers associated with Chaos, such as mutation, daemonic possession, and spiritual damnation, some might wonder why anyone would willingly choose to serve the Ruinous Powers. The answer is actually simple. Chaos judges its servants solely on their abilities and their records of success and failure in promoting the agenda of the individual Chaos Gods and of Chaos Undivided as a whole. Chaos also offers those who serve it the opportunity to perhaps one day wield power and respect far beyond the dreams of most mortals' avarice. For so many people, but particularly in the oppressive, feudal Imperium of Man, where too often family connections and inherited wealth are rewarded rather than ability, and where it is impossible on many worlds for a talented individual to ever better their lot, Chaos actually offers a substantial degree of freedom and the only real meritocracy that they have ever known. But Chaos is not a benevolent force, and for the vast majority of those who fall to Chaos' temptations, they will never no either power or position and will be little more than faceless puppets in the endless schemes of the Dark Gods.
A Chaos Space Marine, also sometimes called a Traitor Marine, is a former Loyalist Space Marine of the Imperium of Man who has chosen to betray the service of the Emperor of Mankind and dedicate his soul to the service of Chaos and its foul deities. Such corrupt Astartes normally belong to one of the 9 Traitor Legions that betrayed the Emperor during the ancient civil war called the Horus Heresy more than 10,000 standard years ago, while others come from Space Marine Chapters created long after the Heresy ended. Chaos can corrupt Astartes of any time and place just as its siren call leads many lesser men to their damnation. The Chaos Space Marines have the same genetic origins as the Space Marines in the gene-seed created by the Emperor from the genomes of the 20 Primarchs, and thus have the same physical abilities as their uncorrupted counterparts. Due to their allegiance to Chaos, Veteran Chaos Space Marines often suffer from extreme mutations (or Chaotic "gifts" depending on the point of view). Traitor Marines are virtually immortal due to their sojourns within the Warp, and their millennia of experience afford them levels of tactical mastery and advanced battlefield skills that the much younger Loyalist Space Marines have not yet have earned.