Placing an Corn in the crafting grid gives you a Corn Seed.
To farm Corn, you plant and grow the seeds (or the Corn itself) on tilled soil, once they're fully grown you can right-click them, harvest the Corn and let them grow again. When harvested by right-click, the plant doesn't break, and yields 1-3 Corn.
Shapeless Crafting Edit
Here are all the recipes using Corn: