A post hole clam-shell digger, also called post hole pincer or simply post hole digger, is a tool consisting of two articulated shovel-like blades, forming an incomplete hollow cylinder about a foot long and a few inches wide, with two long handles that can put the blades in an "open" (parallel) position or a "closed" (convergent) position.
The tool is used to dig holes in the ground, typically from a few inches to a about a foot in diameter, for general purposes such as setting fence and sign posts or planting saplings.
An earth auger is another tool that is used to dig holes in the ground, consisting of a rotating shaft with one or more blades attached at the lower end.
A hand-powered auger is generally easier to use than a clam-shell digger, and can in principle dig deeper and remove more dry, sandy soils.
Clam-shell-type pole diggers seem to be a relatively recent invention, newer than earth augers.