Print head for an
IBM Selectric typewriter (and
clones).
A conventional typewriter has a lever connected to each key; striking the key forces the lever against the ink ribbon, where the letter imprinted at the tip of the lever makes its shape appear on the paper. Clearly, replacing the font or character set is a major undertaking -- all levers must be retooled.
The golf ball is ellipsoidally shaped, and has all the letter imprints on it. This makes it look a (little bit) like a golf ball. When you hit a key, the typewriter rotates the golf ball (along 2 axes!) so the correct imprint is facing the paper, then strikes the entire ball against the ribbon and paper. Replacing the font or character set is trivial -- just pop off the old golf ball and pop in a new one.