A watercolor pan is a small block or cake of dried watercolor paint. To use it, you simply wet the pan of paint with a damp brush; the water moistens the pan, and you get fluid watercolor paint. How strong the paint color is depends on how much water you've used and the extent to which you've rubbed the brush against the dried block to 'dissolve' the paint.
As you use the paint, the pan will develop a hole or well in it (as in the dark blue pan in the photo). You can replace a used pan in a set with a new one, or by squeezing out some tube watercolor paint into the empty pan (the paint will eventually dry out, like a 'normal' pan).
As with all paint, you get students' quality and artist's quality watercolor pans. It's a myth that artist's quality watercolor comes in tubes only, or that tube colors are more saturated.
Watercolor pans can be bought as full pans or half-pans, individually or as as a boxed set.
