What is a crater?
A crater is a round hollow in the ground made when a fast space rock smashes into a planet or moon. Volcanoes can make craters too.
A crater is a bowl-shaped hole, usually round, with a raised edge called a rim. Many craters are made when a space rock hits the surface of a planet, moon or asteroid at very high speed. Others are made by volcanoes, but in this answer we are talking about impact craters.
When a big space rock hits the ground, it is going so fast, many kilometres every second, that it does not just make a dent. It blasts the ground apart like an explosion. Rock is thrown out in all directions, leaving a round hole. Even if the space rock comes in at a slant, the crater is usually round, because the blast spreads out evenly.
Craters come in all sizes. Some on the Moon are smaller than a footprint, made by tiny dust grains. Others are hundreds of kilometres across. Big craters can have a mountain in the middle, called a central peak, which forms when the ground springs back up after the impact.
You can see craters on the Moon with binoculars. On Earth, a famous one is Meteor Crater in Arizona, about 1.2 kilometres wide. Scientists study craters to learn how old a surface is: the more craters, the older it usually is.