There are many craters on the moon which are formed when something smashes on the surface and causes a dent. These craters are formed mainly because of the collision of asteroids, meteorites with the moon. They strike on surface with different speeds but the average speed is 12 miles per second or twenty kilometers per second. Earth surface also have craters but not as much that on the moon because earth has the atmosphere which burns up all these things as soon as they enter the atmosphere. Moon does not have the atmosphere so small rocks hit the surface and has much more craters than the earth. Earth also has erosions, winds, rain while the moon has not. That is why it stays longer on the moon. So they cannot change until another new object strikes the moon. Craters on the moon are called impact craters.
Craters can be of small size or they can be hundreds of kilometer long. The biggest craters are filled with lava and only their outlines are visible.
There are many types of craters on the moon depending on the speed, mass, angle of falling and size of the object striking moon. When an object of small size and with slow speed strikes then simple craters are formed. When objects with fast speed and large in size hits cause complex craters but when very big objects with enormous speed strikes it forms a big crater and the ejected material again falls on the surface forming new craters.