What is a grain?
A Grain is the fruit or seed of cereal grasses. The natural (unprocessed) kernel of grain consists of the germ which is surrounded by the endosperm and bran and covered by the hull. You won't find fields of wheat or corn in the wild as we know them because they never existed and cannot exist in the wild.
The grains we use for food exist today, as we know them, because of a process called neoteny. Neoteny is the retention of juvenile characteristics in the adults of a species and is acquired through gene mutation/selection.
As a result of human manipulation (selection for those grains that didn't mature) the grain doesn't drop it's seed (the whole grain) but instead retains it. Unable to reseed itself the grain would cease to exist without human intervention.
Grains today are milled; bran ¹, ², and germ, which contains 90% of the nutrients of the kernel, are removed leaving only the endosperm (starch). The starch is ground further into different sizes for different products. The result is nutritionally deficient refined flour.
In the U.S., products containing whole grains must indicate on the label that they contain: "WHOLE WHEAT FLOUR" OR "WHOLE GRAIN FLOUR" . Products labeled "7 Grain", "12 Grain", Bran, Rye, etc. usually are made from "refined" and not "whole grain flour".