Gouache is, apparently, an underrated yet extremely versatile medium, according to the Artists & Illustrators Magazine.
Gouache paint is often called 'Designers Gouache' due to it's opaqueness, speed of drying and matt finish, being so necessary for designer, illustrators and commercial artists. It is also often referred to as 'opaque watercolour'.
A good quality Gouache paint is a mix of pigment and Gum Arabic. Gum Arabic is the binder for the pigment, giving it a creamy 'flow' consistency, cheaper varieties may have chalk added to the pigment and they often 'flow' less than a better quality product. Gouache can be used thin to give a wash at the start of a painting allowing the thicker layers to be used towards the end.
There is a lovely article in the June 2011 edition of the Artist & Illustrators Magazine about an artist called Richard Tuff. I've created a link so that you can see his "Across the River" exhibition - just click on his name in the previous sentence.