Watercolour paints are made of transparent pigments suspended in a water-based solution using a gum arabic binder revealing the paint support. Watercolour paint is generally sold in solid blocks called pans and half-pans, although you can buy it in tubes of liquid paint. The two main techniques used are dry or wet.
This is a very old technique which became popular during the Renaissance through botanical illustrations, then used by many Dutch and Flemish artists.