WIKI is pretty good http://en.wikipedia.org/wiki/Smith's_fracture
Smith = reverse Colles' fracture is a fracture of the distal radius.[1] It is caused by a direct blow to the dorsal forearm[2] or falling onto flexed wrists, as opposed to a Colles' fracture which occurs as a result of falling onto wrists in extension. Smith's fractures are less common than Colles' fractures. The distal fracture fragment is displaced volarly (ventrally), as opposed to a Colles' fracture which the fragment is displaced dorsally. Depending on the severity of the impact, there may be one or many fragments and it may or may not involve the articular surface of the wrist joint.
My mneumonic --
Colles = dinner fork
Smith = the other way