An asteroid is a small rocky mass of substance that orbits around the sun. They are smaller than a planet in size but larger than a pebble sized mass called meteoroids.
Sometimes a meteoroid comes close enough to the Earth and enters the Earth’s atmosphere, due to friction with the atmosphere it vaporizes and turns into a meteor appearing as a streak of light in the sky called meteor.
Comets are the mass of ice and dust revolving around the sun. They keep coming closer to the sun in a helical path and when there is enough heat these get vapourised leaving behind a tail of vapour and dust.
When one asteroid smashes into another breaking off small pieces which are called meteoroids if they get completely burn into ashes or vapourize when passing through the atmosphere of the earth, but if they fall as a mass of rock they are known as meteorites.
A planetesimal is a solid object that arises due to the accumulation of orbiting bodies whose internal strength is dominated by self-gravity and whose orbital dynamics is not significantly affected by gas drag.
