Whole corn, like you eat on the cob, is considered a vegetable. The corn kernel itself (where popcorn comes from) is considered a grain. To be more specific, this form of corn is a “whole” grain. To complicate things a little more, many grains including popcorn are considered to be a fruit.
Corn stover consists of the leaves, stalks, and cobs of maize (corn) (Zea mays ssp. mays L.) plants left in a field after harvest. Such stover makes up about half of the yield of a corn crop and is similar to straw from other cereal grasses; in Britain it is sometimes called corn straw.