Gouache is an opaque watercolor paint. Whereas transparent watercolors allow you to see the "white" of the paper below the paint, gouache paint can be applied in solid colors, which allows artists 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. Acrylic gouache is like traditional gouache in that dries to a matte finish and is opaque. Unlike traditional gouache, it has an acrylic binder to make it water-resistant once dry. It can also be painted on a variety of surfaces other than canvas and paper.