This image shows a simple crater on Mars. You can tell it is a simple crater because it is bowl-shaped with no central peak.
This image shows a complex crater on Mars. You can tell it is a complex crater because of the central peak. You can also see where the sides of the crater slumped down when it was formed, and some ejecta around the crater.
This video shows a glass bead being dropped into sand to form a crater. You can see the ejecta flying upward and landing around the crater. The crater left behind looks like a simple crater.
This video shows a water droplet hitting the surface in slow motion. You can see the water's surface push outward then come back up in the middle when the droplet hits. This models the formation of complex craters, and shows why they have a central peak in the middle.
This article gives a lot of background information on craters, including how an impact crater forms, the difference between simple and complex craters, and where impact craters can be found on Earth. I chose this article because it covers a wide breadth of topics on impact craters and is not so technical that it is hard to understand.