Gouache is an artists paint. Often also called 'designers paint' and 'poster paint'. It was traditionally used alot by graphic designers well before the invent of computers, when posters incorporated hand painted illustrations.

Gouache is soluble in water, it has a chalky base, and dries with a flat color. Gouache can be handled in a similar way as watercolor paints, however gouache is very much opaque, it is not transparent like watercolors. Gouache allows dry layers of color to be overlapped without showing a color mix due to transparency.

With the invention of polymer paints, some gouaches have become advanced, and have evolved to become a hybred type of acrylic paint. Brands such as Jo Sonja's have an acrylic base, though still dry to a highly opaque flat finish, once dry they are no longer soluble. For many artists this has greatly improved upon traditional gouaches and have opened the door for new painting techniques.

For further links & information on Artists Paint see: Artists Paint Brands.