There are two main types of seismic waves: one is body wave, which travels through Earth's interior, and the other one is surface wave, which travels along the Earth's surfaces. We can also categorize the body wave into two types. One is compressional P wave. The other is shear S wave. Figure 1 shows a demonstration of horizontal propagations of the body waves.
At first, please pinch an open circle on the horizontal middle line. Then move it to the right, and release it. You can find a "red" wave crest, which propagates to the left. The particle motion is parallel to the propagation direction. This is P wave.
Next, please move it to the downward. You can find a "green" wave crest, which propagates to the left. The particle motion is perpendicular to the propagation direction. This is S wave.
Please move an open circle to an oblique direction. You can find both P wave (red) and S wave (green). You can also find P wave travels faster than S wave. Originally, P wave is abbreviation for primary wave (the first arrival at a station), and S wave is that for secondary wave.