An asteroid is a randomly created celestial body with sizes between 3 and 30 meters.
This is the first type of celestial body that is not on rails to change their orbits. This means that they can be captured by other bodies or collide with them. The current Kerbol System is designed to prevent any SOI change which would disturb their orbit. Unlike the other celestial bodies, asteroids are too light to have any gravity. This makes them more like a non-player created craft. But they need to be discovered and monitored with the Tracking Station. They come in various classes from class A (tiny) to class E (Huge). Asteroids can be white, black and grey and come in random shapes.
Asteroids should not be confused with captured asteroids, as they are on rails and have gravity.
Currently only samples can be taken from each asteroid yielding 60 Science. However, different samples can be collected from the same asteroid in different situations, like high orbit, low orbits, atmosphere and even in different biomes when the asteroid is landed on a celestial body.
Asteroid Redirect Mission
Along with asteroids, the official Asteroid Redirect Mission has been released containing a mission to tug an asteroid into Kerbin orbit to investigate it. The asteroids are lightweight and not on rails to allow their orbit to be altered. It is believed (unconfirmed) that the asteroids are parts, as opposed to celestial bodies.
- Asteroid on Wikipedia