Asteroids are any objects orbiting the sun
that aren't planets or comets.
Asteroids are mainly made of mineral and
rock, whereas comets are made of dust and ice.
However, the difference between planets and
asteroids is changing all the time, and is the
subject of debate!
You may know that Pluto used to be called a
planet, but it was demoted and is no longer
considered one, for example. Asteroids tend to
form near the sun, and if they're big enough, the
gravity they generate tends to be enough to cause
them to become round like we think of planets. We
tend to call these asteroids dwarf planets.
So Earth is nothing more than a big asteroid!