Watercolor or water-soluble pencils and crayons are a unique cross-over between drawing and painting. You draw with them as you would with any pencil or crayon, but then if you run a wet brush over your drawing, the color is dispersed and turns into a watercolor wash. They have the advantage of being easy to use, relatively cheap, and don't leave you with a mess to clean up.
Watercolor pencils are specifically manufactured with a binder that dissolves in water.
What types of watercolor pencils are available?
Water-soluble pencils are available in a wide range of colors, as well as plain graphite pencils. Colored watercolor pencils aren't graded like graphite pencils are (from 9B, the softest, to 9H, the hardest), but their softness does vary between brands so it may be worth buying a sample pencil from various brands to see which you prefer before you buy a set. The softer a watercolor pencil is, the easier it is to put color or pigment down on a paper. (See Best Water-Soluble Pencils & Crayons.)
Woodless Watercolor Pencils and Water-Soluble Crayons
Two variations on watercolor pencils available are woodless pencils (just the pencil 'lead' with a paper wrapper) and water-soluble crayons (like wax crayons, but they dissolve in water). Water-soluble crayons enable you to put down more pigment (or colour) faster than a watercolor pencil, as they're softer and broader.