Chaos is a part of the psychic energy that composes Warp space. More generally, "Chaos" refers to anything related to Chaos, including its influence, the Gods of Chaos, their followers, and the Warp. Chaos is almost synonymous with the Warp - the two concepts are inseparable: Chaos is the limitless ocean of spiritual and emotional energy that defines the Warp. It is a great and raw force of change and power, and is both physically and spiritually corrupting. The most gifted mortals, psykers, can utilise this energy, thus making them capable of abilities which transcend the laws of the material universe. However, the malevolent power of Chaos can gradually corrupt a psyker, tainting their mind and body.
The forces of Chaos are those who have pledged themselves to the Gods of Chaos.