Asteroids are the leftover material from the formation of a star system. They're too small to form a planet and didn't collide with a planet. They can consist of carbon (Type C), metal (Type M) and stone (Type S).
Asteroids that collide with planets can cause mass extinctions such as the extinction of the dinosaurs on Earth.
Dozens of asteroid bases have been built inside big asteroids. They're located in deep space.