There are many different types of corn, with the two most common types being sweet and field corn. Within these two main groups are many distinct and separate varieties, which are often categorized based on the shape of the kernel. Each variety has its advantages and disadvantages: some are bred for particularly high yields, while others are bred for their taste, and still others are bred for a particular shape. There are also specialty corns, some of which are grown mostly for experimental use or for decoration.
Sweet corns are those that can be eaten by humans. The most common types are standard sugary, augmented sugary, shrunken, and brittle. There are also supersweet varieties that have been bred to be much sweeter than regular ones and to have a longer shelf life than other varieties. All of these have a distinctive kernel shape, with a very sugary top covering the hard bottom of the kernel, which holds the germ. Farmers choose which varieties to plant based not only on taste, but also on local conditions: some work better in windier conditions, for example, while others are good for short growing seasons.
There are also many varieties of field corn, with the most common being dent, flint, flour, and waxy. The kernels of each of these types all have unique features. For example, flint varieties have a hard outer layer that protects the entire kernel, while dent corns have dents in the crown of each kernel when it is fully ripe and beginning to dry. The kernels of flour corns are mostly made up of starch, and have a very thin outside. Waxy ones have a unique genetic structure that makes them very useful for making starches and adhesives.
Within these broad subdivisions there are a number of specialty corns, including those with colored kernels, popcorn, and corns that are bred to have particularly high levels of certain nutrients. Blue corn, a type of the flour variety, is one of the most well-known specialty corns, but there are many other types, including multi-colored corns known as Indian corns, which are actually a type of flint corn.
There is also popcorn, which has a distinctive closed kernel shape that allows moisture to build up until the kernel explodes, making it edible. Varieties have been bred for nutrition, texture, and even specific shapes of the popped kernel. There are also many other varieties that have been bred to be particularly high in a certain component, like lysine or amylase. These are often made for a particular purpose: for example, high-lysine corn is bred to fulfill the nutritional needs of pigs.
In general, most of the world's harvest is made up of field corns. These can be used for livestock feed, for making alternative fuel, alcohol, solvents, and even fibers. One variety, pod corn, is used almost exclusively for decoration, since each kernel is encased in a hard covering that makes it difficult to use for other purposes. Since pod corns are also very similar to primitive varieties, they're commonly used for research purposes.