Four major layers make up the earth. The center of the earth is called the inner core and outer core. Above this is the mantle and then the crust. The crust fits around the earth in a type of puzzle-piece fashion. In other words, the crust doesn't cover the earth in one unbroken manner. It is made up of plates, which scientists call tectonic plates, that fit together to complete the crust. These plates aren't stationary, either. They move about as fast as your fingernails grow, compelled by movement happening from underneath the earth.
The movement that creates an earthquake occurs at fault lines. A fault line is what scientists call the place where two tectonic plates meet. As tectonic plates move past each other in separate directions they sometimes snag. This is generally the result of jagged spots on the plates. The longer the plates remain snagged the more energy they build up. The pressure against the plates becomes too great and they break apart. The stored energy emanates out from the fault line, now called the hypocenter, creating an earthquake.
A normal fault occurs when one tectonic plate moves downward and away from another plate. This also occurs when a plate splits and part of the plate pulls down and away from the other part of the plate. Each plate, or each part of the plate, usually slants. Imagine this as a piece of rock falling off of a cliff. A reverse fault occurs when one plate or one piece of a plate pushes against the other instead of pulling away. Imagine that the edge of a cliff, instead of falling away, pushes up against the cliff, creating a portion that you would step up onto. This is what a reverse fault looks like.
A strike-slip fault occurs when two tectonic plates move past each other horizontally. These come in two types: left and right strike-slip. When you are experiencing a left strike-slip, the plate you are not standing on always moves left. This is true for a right strike-slip as well, only backwards. The most famous strike-slip fault is the San Andreas fault in California.