Placing an Barley in the crafting grid gives you a Barley Seed.
To farm the Barley, you plant and grow the seeds (or the Barley itself) on tilled soil, once they're fully grown you can right-click them, harvest the Barley and let them grow again. When harvested by right-click, the plant doesn't break, and yields 1-2 Barley.
Shapeless Crafting Edit
Here are all the recipes using Barley:
Note: If you're running this under FTB Infinity, you have to use the HarvestCraft Barley and not the one from Natura to make Flour.