Grout is used to fill voids, and seal joints such as those between tiles to create a smooth, attractive water resistant surface. It is generally composed of a mixture of water, cement and sand, often colour tint, and sometimes fine gravel. It is applied as a thick liquid which hardens over time, much like mortar. If the grout is crumbly, or powdery and falling out especially in bathrooms water may penetrate through the tile joints and cause damage to the substrate or the building structure. Crumbly or powdery grout is due to poor mixing and application.

WIKIPEDIA ENTRY FOR GROUT.