Asteroid Belt

The Asteroid Belt isn't a planet but is rather a distinct region of the Solar System sitting between the orbits of Mars and Jupiter around the Sun. It is composed of many different irregularly shaped asteroids but these are relatively small, as the total mass of the belt is roughly 4% of the Moon's entire mass(7.342 * 10²² kg). The most notable object in the main asteroid belt is Ceres because it is large enough to qualify as a dwarf planet.

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Just for reference, an asteroid is essentially a minor planet in the inner Solar System. However, any object that isn't entirely a planet but doesn't exhibit comet characteristics(like volatility and having a tail) is considered an asteroid if small enough. In contrast, a comet is a body that releases hot gases when passing by the Sun and is fairly volatile in nature.

The main asteroid belt is the region of the image above that is white and between Mars and Jupiter. The belt likely formed as planetesimals(small/minor planets) from the solar nebula about 4.5 billion years ago. These would've accreted into protoplanets(a body about to develop into a planet) but due to Jupiter's strong gravitational pull, this was never allowed gravitationally.

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There are three main asteroid types in the main belt. There are C-type(Carbonaceous) asteroids that are heavily carbon-based(as suggested by the name), reddish, and comprise over 75% of the visible asteroids in the belt. Next up are S-type(Silicate) asteroids in the inner belt mainly composed of silicates. Lastly are M-type(metal-rich) asteroids which are composed of metals from nickel to iron mainly.

Since these Kirkwood Gaps, as they are called, are in locations where there would be orbital resonance(the ratio of orbital periods is an integer fraction), no asteroids orbit in these gaps. This is represented by the distribution above because there are little to no asteroids in the gaps of the distribution where orbital resonance occurs. While some come close on smaller time scales due to highly eccentric(elliptical) orbits, they eventually smooth out because they come too close into contact with a major planet like Mars or Jupiter and move into different orbits.