Barley and wheat are two different types of grass, both in physical structure and the uses for each. Barley is a yearly grass with shorter ligules on its blades and smoother sheaths, while wheat has a hollow rachis.
Breweries use barley more often to make beer than wheat typically because the barley has a stronger flavor, which makes a more robust beer. Barley has more fiber than wheat, and it's easier to cook because it doesn't require milling like wheat does. Wheat is typically used more in breads, and it makes a cost-effective feed for livestock. Another difference is the time of year in which these two grasses thrive. Barley does best in a warmer climate, which is why it is usually harvested during spring and summer. Wheat does best in cooler climates, so farmers harvest it in the fall.