What type of wave is an S wave?

An S-wave is a type of seismic wave in an earthquake.

In an S-wave, the medium is displaced in a transverse (up and down - compared to the line of travel) way, and the medium must shear or "move away" from the material right next to it to cause the shear and transmit the wave.

S waves are secondary waves that vibrate from side and side as well as up and down. S waves shake buildings violently and cannot move through liquids.

See the related questions and related link to Wikipedia for additional information.
s-waves are secondary waves first come primary wave the secondary wave and then surface wave.