They are found randomly when breaking tall grass, much like vanilla wheat seeds. When breaking a Corn stalk, 0-3 Corn Kernels are dropped.
Corn Kernels can only be planted on watered (dark) farmland and will pop off on dry soil.
They can be planted to grow Corn. In addition, they can be cooked into Popcorn for eating purposes.
It is possible to plant Corn Kernels on dry farmland, however any block update will cause the Kernel to pop off.