http://animals.pawnation.com/difference-between-hay-straw-bedding-7251.html says:
Hay is a type of forage commonly fed to horses, cattle and other livestock. Hay is grown primarily for its nutritional value, and it is cut before the plant can start producing seeds that would lower its nutritional benefit. Hay provides high-quality fiber as well as other nutrients that animals need to stay healthy.
Straw is the stalk of the plant after the grain or seeds have been harvested. Straw is not grown for the sake of producing straw, it is the byproduct of other types of farming. Plants that produce straw have already put a significant amount of their available nutrients into the grain or seed they produce; straw is the leftover stalks of the plants after harvest. Straw provides low-quality forage.
Straw is normally used for a bedding material, while hay is used as a feed supplement. Straw is dried out and works well for building nests and providing cushioning for animals to sleep in. It is not moist like hay and is unlikely to mold. Straw is also more practical to use as bedding than hay is, because of its lower cost.
I only mentioned it because I've seen so many of the more experienced chicken keepers on here "warn" against hay as bedding. Until I started frequenting BYC, I never really considered the difference either.
