A wave in which the disturbance is an elastic deformation perpendicular to the direction of motion of the wave. A wave in which the disturbance is a compression of the medium. In the animation you may choose to see either shear waves or compression waves. Observe that for shear waves, the motion of the medium is up and down, perpendicular to the direction the wave is moving. For compression waves the motion of the medium is back and forth parallel to the direction the wave is moving.
Earthquakes can produce both kinds of waves.
Each kind moves through the interior of the earth to be detected by seismographs on the other side of the earth. However, the compression waves travel faster than the shear waves.