There are 4 types of corn. Dimple/dent corn, pop corn, sweet corn, and flint corn. Each variety can be eaten. Prepared differently of course as they have different starches and flavors but the vast majority of corn fields in the United States grow dent corn for feed and biofuels.
Heirloom is just corn passed down from your pop-pop ;) (though the category now stands for rare varieties of corn that came from family farms or from the years long since passed).
Corn is amazing. My favorite use of it is for alcohol though.