Most faults are under the earth's surface, but there is one that can be clearly seen. It is in the United States in California. It is called the San Andreas fault. Here, the edges of two plates are sliding past one another, and the deep ridge between them can be seen. It looks like a long crack or scar on the earth's surface.
Earthquakes often happen along faults.
If the force is too great, the rocks that make up the plates cannot stretch anymore. The rocks break apart, causing an earthquake. As the built-up pressure in the snapping rocks is set free, the rocks bump against one another, and the earth shakes. Think of how a tightly pulled rubber band snaps and then shakes when it is let go. Something like that happens during an earthquake, only in a much larger way.