Art pens and markers cover a broad spectrum of possibilities. Essentially a pen is a hand held device containing colour in various forms, ink, acrylic, alcohol and watercolour. Pen nibs also come in a huge range which dictate the resulting mark on the surface. Fine liners are available for intricate fine markings to broad chisel tipped hard sponge tips which allow for huge amounts of pigment to be delivered in a consistent stroke.
Pens lend themselves to illustration, it is the consistency of the marking that makes this tool a fantastic artist asset.