A stook, also referred to as a shock or stack, is an arrangement of sheaves of cut grain-stalks placed so as to keep the grain-heads off the ground while still in the field and prior to collection for threshing. Stooked grain sheaves are typically wheat, barley and oats.The Oxford Dictionaries website lists the etymology as Middle English. When I was growing up in Minnesota, we would refer to these as "shocks," reserving the word "stack" for the larger haystacks.
The purpose of a stook [or 'stooking'] is to dry the unthreshed grain while protecting it from vermin until it is brought into long-term storage. The unthreshed grain also cures while in a stook. In England, sheaves were commonly stacked in stooks of twelve and may therefor refer to twelve sheaves.
In North America, a stook may also refer to a field stack of six, ten or fifteen small (70–90 lb (30–40 kg)), rectangular bales of hay or straw. These bales may be stacked and deposited by a "stooking machine" or "stooker" that is dragged, sled-like, behind the baler.
Related: Sculpting with straw and Making hay while the sun shines (video).
Credit for the photo of a field in Devon: Paul McLoughlin, via a gallery of landscape photos at The Guardian.