Gouache is an opaque watercolor paint. Whereas transparent watercolors allow you to see the "white" of the paper below the paint, gouache can be applied in solid colors. This allows an artist to paint in layers from dark to light.
Gouache dries to a matte finish, which makes it easy to scan or reproduce electronically, since there is no glossy shine. Designer's Gouache traditionally offers colors blended from a number of pigments, but some lines of Artist's Gouache offer single-pigment colors. Student Gouache will have working characteristics similar to Designer's Gouache, but with lower pigment concentration, less expensive formulas, and a smaller range of colors.