Corn is a generic term to describe the fruit (grain) of cereal plants in particular. In Britain corn generally refers to wheat. However, in the US, the word corn refers to maize. Popping corn is just one of the many varieties of maize grown commercially.
The bulk of tissue within a grain of corn is called the endosperm. Endosperm is specialized storage tissue providing nutrients for the embryo when the seed germinates. It is also a source of carbohydrates for humans. In popcorn, the outer part of the endosperm is hard but the center is soft. When the corn is heated, the water in the central part turns to steam causing the seed to burst (the pop that you hear) and turn inside out.
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