Chaos was said to be the source of all evil in the galaxy. It is the final villain Sailor Moon had to face.
In the anime, it was sealed away by Sailor Galaxia and eventually managed to corrupt her, but not before she sent her Star Seed, the Light of Hope, out on its own to find "the stars of love and justice."
Eventually it manages to find Sailor Moon, and takes a form that resembles her. This form is Chibi Chibi. In the end, when Sailor Moon manages to free Galaxia on her own and expel Chaos from the body of the powerful Soldier, she explains to Galaxia that no one person should try to fight Chaos alone, but that it should forever be held partially in the hearts of all people where it can not overwhelm.
Chaos is an interesting villain that breaks the barriers and precedent set by all previous Sailor Moon enemies. It is a force of evil in and of itself that manages to corrupt a host who wishes to banish it from existence. Unlike enemies from previous seasons; who all had vendettas, plans, reasons, and/or a more complicated method to their evil; Chaos wasn't willing anything to happen. It was a dormant force of evil that inspired evil in the body and mind of the Golden Senshi who tried to destroy it, while it is a thing that can not be erased from the world as long as humans exist. Interestingly enough, Chaos Galaxia was gathering starseeds, essentially eliminating all human life, in order to control the universe.