Popcorn is a type of corn which explodes and puffs up when it is heated. Popcorn kernels have hard, dry shells, but moist interiors. When heated, the moisture inside boils and converts to steam, increasing interior pressure. This causes the shell to crack open, allowing the inner material to burst out. As the pulp comes out, much of the steam is released, but gas pockets in the pulp cases it to cool and dry puffy.
The American Indians introduced it to the Pilgrim settlers, although adding butter, salt, and other toppings to popcorn came about later on.