The prepackaged ingredients are called PicoPaks. PicoPaks cost roughly $20 and produce five liters of beer. They consist of malted barley, hops and yeast -- the main ingredients normally used when making beer.
After a couple of hours, you'll have unfermented beer -- called wort. Cool it and add yeast to turn your wort into beer. You'll need to wait a week or so while your beer ferments and carbonates, but then you'll have your final product.