A post-hole digger is a contraption designed specifically for digging holes in which to put posts. With me so far?

A post-hole digger is basically a pair of shovels attached by a hinge near where the handle meets the blade. The blade of each shovel has a flat end, end each blade forms a section of a circle.

To use a post-hole digger, hold one handle in each hand, with both shovel blades facing down. Bring the handles together, then jam the two shovel blades into the ground. Pull the handles apart, and lift the chunk of dirt up and into a convenient pile somewhere else. Repeat as necessary.