Process o radioactive dating
The longer the surface has been exposed to space, the more craters it will have.If you know how frequently craters of a given size are created on a planet or moon, you can just count up the number of craters per unit area.Victoria Crater on Mars as seen by the Mars Reconnaissance Orbiter. It is a simple crater that has been partially filled in with sand---note the sand dunes on its floor.
Large craters will have a central peak formed by the rock beneath the impact point rebounding upward and they may also have terracing of the inner walls of the crater from the collapsing of the crater rim inward.
The explosion is what carves out the crater so almost all craters are round (otherwise the great majority would be oblong in shape).
See the "Not Round" page from the THEMIS site for what can make an impact crater not round (links will appear in a new window).
For example, the highland regions on the Moon have ten times the number of craters as the maria, but radioactive dating (explained in the next chapter) shows that the highlands are approximately 500 million years older than the maria, not ten times older.
At a minimum crater-age dating can tell you the ages of surfaces (which surface is older than another).