A "stick figure" is a style of drawing human beings (or other creatures) in which major portions of the body are simplified into single line segments. The majority of xkcd is drawn in a stick-figure style.
A typical stick figure has one line each for the torso, each arm and each leg, and a circle for a head. More complexity can be added to stick figures by adding joints (two line segments for each arm or leg, or a neck) as well as adding line segments for feet, circles for hands, line segments for fingers, or facial features for expression. xkcd typically makes use of these more versatile methods of adding expression or movement to stick figures, although modern xkcd strips rarely use facial features on stick figure characters.
In most stick figure drawings (including xkcd, typically), most other objects depicted are generally drawn in a similarly simplified style, just as simplified outlines or with line segments or basic shapes in place of more complex figures.