Posole is very dried, large kernels of corn.
It is made by soaking harder varieties of corn kernels (usually flint corn) in lime water until swollen, and then allowing them to dry out for storage.
Posole corn can come in every colour that corn does: yellow, white, red and even blue and purple. Usually, though, white or blue corn is preferred.
Hominy is somewhat like posole, but softer and without posole’s more robust taste. Hominy is generally made from dent corn, while posole corn is generally made from flint corn.
You can buy it in its dried form, or canned and ready to use.
To use posole, soak it overnight, then discard the soaking water and rinse. Put in a cooking pot, and cover with a lot of water — cover with twice the depth of water as there is posole. Simmer slowly for about 6 hours until most of the kernels have opened up, then use as per your recipe. The kernels won’t go mushy, they will stay firm. Some recipes may have you add other ingredients such as meat at the start of the cooking process.
The soaking in lime water is an ancient tradition which releases more of the nutrients in the corn. Corn heavy diets which didn’t treat the corn like this left people susceptible to pellagra. See Corn entry for more information.