A shear wave(S-wave) is a type of seismic body waves that shakes the ground back and forth perpendicular to the direction the wave is moving.
Seismic waves are elastic waves usually generated when there is a disturbance within the earth.
There are two types of seismic waves:
Surface waves
Body waves
Body waves travel within the earth and they cause disturbances there. P and S waves are the two types of body waves that we have.
Surface waves travels on the earth surface. They are the love and rayleigh waves. They are the ones that cause destruction on the earth surface during an earthquake.