Who are the over factions in The Expanse universe ?

Earth

Earth, also known as Terra or Sol Ⅲ, is the third planet of the Sol system, and one of its four terrestrial worlds. Home-world of humanity, Earth is the greatest hub of human politics and, technically, still the capital planet and the dominant superpower, with its governing body the United Nations. It has always been at the forefront of human history.

Mars

Mars is the fourth planet from Sol and the second smallest planet in the Sol system. It has a population of ten billion humans,[1] and is governed by the Martian Congressional Republic.

Belter

Belter is a term used to refer to persons born in the Asteroid Belt or the Moons of the outer planets. In a pejorative meaning, they are also called Skinnies, because of their altered physiology due to growing up and living in a low gravity environment.