Hay is mowed from a meadow. It’s fine natural dried long grass with occasional herbs and flowers. It’s still greenish.

Straw is the rough homogenous stalk leftovers of mowing agricrultural plants like rye or wheat.

Hay is for fodder; straw is for bedding and thatching.

Hay is generally cut from fallow fields in late summer/early fall. It contains sugars in the stalks as well as the seeds/grain of whatever grasses it is cut from. It is used to feed livestock year round and is the primary feed in winter. It is often supplemented by grain or feed.

Straw is basically an industrial byproduct and is the leftover stalk from grain harvesting. Usually produced from wheat after harvest. Used as bedding.

Hay is basically cut grass, straw is the part leftover from harvesting wheat and taking the seeds. Both are baled, but they’re used for different things. Hay is food for any animals that eat grass like horses and cows, buy straw is not edible so it’s used as bedding.