Grout is a construction material used to embed rebars in masonry walls, connect sections of pre-cast concrete, fill voids, and seal joints (like those between tiles). Grout is generally composed of a mixture of water, cement, sand and sometimes color tint which is applied as a thick liquid and hardens over time, much like mortar.
It is also a component of mosaics. Although ungrouted mosaics do exist, most have grout between the tesserae.
Main varieties include: tiling grout (either cement-based or epoxy), flooring grout, resin grout, non-shrink grout and thixotropic grout.