Stick figure
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, conveying most 'expressions' through the posing of the whole body, arm gestures or even a tilt of the head.
The most common adornment, to differentiate stick figures, are hats or hair. Hats are more common for characters understood to be male, with hair for those understood to be female, but there are some exceptions and possible ambiguities. Clothing, or other bodily adornements, may also be drawn where it suits the context, but the lack of drawn clothes does not apply any degree of nudity (only possibly the depicted scenario). The most undifferentiated stick figure has no hat or obvious hair, and can either be the anonymous character, a named one or even presumed to be the author's avatar, all of whom may be considered implicitly male.
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. Depictions in silhouette will have figures' heads (and any other items) filled.
