S-waves, or shear waves, are a type of seismic wave that move through the Earth during an earthquake. They are slower than P-waves (primary waves) and can only travel through solid materials. S-waves move the ground up and down or side to side, which can cause significant shaking during seismic events. Unlike P-waves, S-waves cannot travel through liquids, which is why they are not detected in the Earth's outer core. Their behavior helps scientists understand the Earth's internal structure and is crucial for earthquake research and safety measures.