Gouache (pronounced gwash) can be explained as an 'opaque watercolour'. Gouache has much more covering power than watercolour, is more easily controlled, and can be reworked.
It is sometimes referred to as Designers gouache, because it is used by designers, illustrators and commercial artists. Gouache is the preferred medium because it dries quickly to a matt finish, being the desired and required effect.
Student gouache may have chalk added to the pigment as a binder, whereas artists quality are a mix of pigment and Gum Arabic.
The Gum Arabic gives the pigment a creamy, flowing constituency.
Gouache may be used on watercolour paper, board, pastel paper, and heavier cartridge papers.