The MOHO is the name given to the boundary between the crust and the mantle. It's existance was initially detected through a discontinuity in the motion of seismic waves from one point to another through the Earth. Motion of the waves along the MOHO is faster than in the crust itself. So beyond a certain seperation it is quicker for the waves to descend to the level of the MOHO, travel along the MOHO and then return to the surface than it is for them to move along in the crust.
The boundary is really sharp. Evidence for this is seen in the seismic data itself as well as inspection of parts of the MOHO that were deposited in Oman during obduction of an oceanic plate.
The difference in speed for the waves represents a difference in refractive index and this is obviously associated with a difference in compositon. Whilst the crust is made up mainly of basalt and granite the mantle is a peridotite based rock, featuring olivine, and some pyroxenes.