If that weren't bad enough, leave it to modern day marketing techniques to popularize something-even worse, light beer. There ought to be a law. Although water is the major ingredient in beer, its source is of limited importance. Some brewers have concerns about mineral content of the water relative to the style of beer they are making. But overall, if it's drinkable and palatable, whether it's from a mountain spring or a reservoir, it works for beer.
Yeast is used for fermentation, converting the sugars of the grain to alcohol.
It's in the malt and the hops that the differences between the methods of the mass producers and the micros are most apparent. The malt flavor is balanced by the addition of considerable amounts of hops, the better and more expensive versions of which have names like Saaz, Fuggles, and Styrian Goldings. The hops add a pleasant bitterness, and act as a natural preservative. The big boys have an imaginative interpretation of what constitutes malt, substituting liberal quantities of other grains, especially corn and rice. With less malt flavoring to balance, the addition of hops is minimal, further limiting the development of a semblance of a true beer taste.