Watercolor (also called aquarelle) is a beautiful media in which pigment is suspended in a water based solution and applied to the substrate, most commonly special paper made for this purpose. Watercolor paper is often made entirely or partially with cotton, which gives a good texture and minimizes distortion when wet. My paper of choice is Arches #140 cold pressed which I purchase in 22″ x 30″ sheets.
Watercolors are usually translucent, and appear luminous because the pigments are laid down in a pure form. Pigments can be in pan, stick and tube form. I have experimented with all and prefer tube.