Gouache is both a technique and a product. The technique, dating back to before the renaissance, refers to the use of white to achieve opacity in water based colors.
Opaque techniques were popularized by the Impressionists and Post-Impressionists in their use of pastel,lithography and wood cuts.
Gouache, the product, was a result of this interest in both opaque and water based products. Designers Opaque Water Color was developed in the 1930's.
Gouache paint applies a very flat matte painted surface as opposed to acrylics which has a shiny plastic looking finish.
Gouache do not permanently dry . If they get wet they will run again. Acrylics will permanently dry.
Gouache is used mainly by students, designers and illustrators because of this flat matte finish.