Gouache is a water based paint consisting of pigment and other materials designed to be used in an opaque painting method. Gouache differs from watercolor in that the particles are larger, the ratio of pigment to water is much higher, and an additional, inert, white pigment such as chalk is also present.

This makes gouache heavier and more opaque, with greater reflective qualities. Like all watermedia, it is diluted with water.

Source: Wikipedia