A technique in rock climbing where the climber wedges part of his/her body into a crack in the rock.
Overwhelmingly, the body part will be a hand or foot. (Occasionally, it might be a head, elbow, knee, the whole body or what have you). A classic example is the perfect, splitter hand crack. Imagine, if you will, an otherwise black, vertical rock face, split by a crack about 3cm wide (that's about an inch). You, the climber, can slide your hand into the crack, and by flexing the hand, cause it to wedge into the crack. Then, by tipping your foot sideways, stuff your toes into the crack and sort of un twist your foot, thus wedging it into the crack also.
You make upwards progress by repeating with the remaining limbs, each a bit higher than the last. Believe it or not, you can actually do this.
The two main modes of free climbing are crack climbing (which is mostly based on jamming) and face climbing.
Jamming ranges in width (based on the width of the crack) from the tips of one's fingers, on to the partial fingers , the whole fingers, the hand, off-hand (where the crack is a bit too wide for a soid hand jam), fist (where you make a fist, stuff it in the crack and squeeze to enlarge your fist and create friction), to off-fist. As the crack gets wider, the climber begins to wedge his/her arm into the crack. In a large crack, the climber can actually get inside the crack and skooch up a bit at a time in what is called chimneying. Between chimneying and jamming is the dreaded zone of the off-width crack (scary, bad things, beloved by certain masochists)