Crimping is a type of hold used in rock climbing that relies only on the fingertips for support. It's characterized by the hyperextension of the first joint in the fingers and a contraction of the second joint. An easy way to picture this is to visualize your fingers bent at your first knuckle while grasping onto a small edge. As a rule, the technique is best used on holds that are no deeper than an inch to avoid unnecessarily stressing your tendons.
Source: Flickr User A_E_P