When a person falls, sticks out her hand and breaks her wrist, which bone (or bones) actually break?

Scaphoid bone: A common bone to break in the wrist with a fall on outstretched hand (f.O.O.H.) is the scaphoid (formerly called the navicular) bone. This bone has a tendency not to heal properly so early care and immobilization is recommended. Xrays may not pick up the fracture and pain/tenderness is at the thumb base ('snuff box'). Other bones in the wrist can fracture, too (e.g. Radius) so xrays are needed.

Distal radius: You typically break the end of your radius (the bone between your elbow and your thumb) in one of two patterns called colle's and smith's fractures. These depend on whether you fall forward or backward. This is the most common fracture of the upper extremity. The picture shows the classic "dinner fork deformity" typical of colle's fracture.

A few possibilities: The only sure way to know which bone is fractured is to have an xray of the hand/wrist. Most commonly, a fall on an outstretched hand will give a distal radius fracture. However, the distal ulna, scaphoid and other carpal bones can all be injured in a fall.