An auger is a drilling device, or drill bit, that usually includes a rotating helical screw blade called a "flighting" to act as a screw conveyor to remove the drilled out material. The rotation of the blade causes the material to move out of the hole being drilled.
An auger used for digging post holes is called an earth auger, handheld power earth drill, soil auger, or mechanized post hole digger. This kind of auger can be a manually turned, handheld device, or powered by an electric motor or internal-combustion engine, possibly attached to a tractor (being provided with power by the tractor engine's power take-off as shown). Handheld augers can also be used for making holes for garden planting.
Wood augers have a screw to pull them into the wood, as a gimlet has, and a cutting lip that slices out the bottom of the hole. The auger bit, meant to be used in a brace, also has cutting spurs to cut a clean circle deeper than where the lips scrape out the wood.
In construction, augers are used for special drilling rigs to dig holes, or augerating for deep foundation piles. Another use is for piles forming a piling retaining wall, which can be constructed in the same way as foundation piles.
Study of a man using an auger, from The Seven Sorrows of the Virgin, Albrecht Dürer, ca 1496
An auger bit in a brace for woodworking.