See C. McCormick, The Century of the Reaper (1931, repr. 1971).
A reaper is a person (or machine) who reaps, or harvests (cuts and gathers) crops when they are ripe.
Reaped grain stalks tied together in a bunch is called a sheaf (plural sheafs or sheaves), and several of these may be stood together to dry out with the ears off the ground, forming stooks. A stack of sheaves may be stored for winter threshing, the sheaves being placed with the ears inwards, then covered with thatch or a tarpaulin; this is called a stack or rick (in the British Isles, where "corn" traditionally means "grain", normally corn-rick, to distinguish it from a hay rick). Ricks would be made in an area inaccessible to livestock, called a rick-yard or stack-yard.
The Romans invented a simple mechanical reaper that cut the ears without the straw and was pushed by oxen (Pliny the Elder Nat. His. 18,296). This device was forgotten in the Dark Ages, during which period reapers reverted to using scythes and sickles to gather crops.
A much more sophisticated mechanical reaper was invented in 1831 in Union Bridge, Maryland, and patented by Cyrus McCormick in 1834 as a horse-drawn farm implement to cut small grain crops. It developed into and was replaced by the reaper-binder, which was in turn replaced by the swather and eventually the combine harvester.