Asteroids represent material left over from the formation of the solar system. Although is has been suggested that asteroids are the remains of a planet that was destroyed in a massive collision, it is more likely that they represent material that never coalesced into a planet. If the estimated total mass of all asteroids was gathered into a single object, the object would be less than half the diameter of the Moon (Ref).
The surface gravitational force is so weak on this small body that the gravitational escape speed is only 10 meters per second---comparable to the speed of a fast sprinter. In this gravitational field, a 200 pound man would weigh only 0.1 pounds!
Gaspra has a high concentration of small craters compared to larger ones relative to other small objects in the Solar System like the moons of Mars (which may be captured asteroids). This is consistent with the theory that Gaspra originated comparatively recently from the collisional breakup of a larger body perhaps 10 times the present size of Gaspra (Ref).
Ida (left) is about 56 kilometers long and Dactyl (right) is about 1.5 kilometers across in this view. Dactyl is actually in the foreground, about 80 kilometers closer to the spacecraft than Ida is. Dactyl is not identical in spectral properties to any area of Ida in view here, though its overall similarity in reflectance and general spectral type suggests that it is made of basically the same rock types (Ref).
However, the asteroid 4179 Toutatis (which crosses Earth's orbit) has been found through radio telescope observations to have an irregular shape and a complex tumbling rotation---both thought to arise from a history of violent collisions. Here is a short animation (47 kB MPEG) of the spin of Toutatis.
It is believed that violent collisions are common for asteroids, and that many asteroids have in their past experienced complex rotations like Toutatis as a consequence of such collisions. However, internal friction has caused them to dampen into simple rotation in relatively brief amounts of time; thus, most asteroids are observed to have simple rotations. But Toutatis rotates so slowly that this dampening process would take much longer than the age of the Solar System. Thus, the rotation of Toutatis may be a relic of the collisional evolution of an asteroid (Ref).