This is a little like saying that a blog would focus on how grapes affect wine. Grains are integral to brewing, and beer itself would not exist without them.
Beer has been known as "liquid bread" and that is essentially what it is. Both bread and beer go through many of the same processes to arrive at very similar ends. Just as a brilliant baker has both art and science to use in pursuit of great bread, so a brilliant brewer uses both in pursuit of amazing beer.
The first grain that we will look at is Barley, the undisputed king of brewing grains. Barley is a short season grain that grows well even in northern climates. This is part of the reason that beer is predominately in cooler climates than wine which needs warmer climates for the grapes. Barley in its raw form is still attached to its husk and is difficult to seperate from it. This makes it less pleasent to eat, and is the reason that barley for eating is often "pearled" or has the husk removed. However, the husk of barley is indespensible for making a good beer.
Barley is first malted in order to be made into beer. Essentially, malting is sprouting the grain, and then drying or roasting to stop the sprout. This allows all of the enzymes to form which convert starch into sugar, but does not allow the plant to use up the sugars. Depending on the roasting process after malting, you can arrive at vastly different colors and flavors of barley malt.
The first of these is base malts. Base malts are so named because they are the foundation for nearly any beer. These malts are dried at very low temperatures so as not to change their color. They provide most of the sugars (that are then turned to alcohol) in any given beer, and provide malty sweetness. Base malts will often create a pale yellow beer.
Next come kilned malts. Kilned malts are roasted at a slightly higher temperature for longer. While still contributing a yellow color (in most cases) kilned malts often have more forward flavors and aromas than base malts. They may contribute toasty, nutty, or graham cracker aromas and flavors to a beer.
Next are caramel malts. In caramel malts, the sugar conversion happens at the maltster prior to the roasting. The roasting then causes caramelization of the sugars. These malts will add a red or orange color to a beer. Their flavors range from toasty, to caramelly sweet, all the way through to dark fruit or raisiny flavors.
The dark malts are the last of the colored malts. These malts are used in small amounts to give a dark color to a beer. In addition to the color, these malts will add dark chocolate, roasty, or coffee notes to a beer. Too much dark malt can contribute an unpleasent astringancy to a beer.
Last are the dextrin malts. These malts are created in such a way as to have a large amount of maltodextrin, a non-fermentable sugar. While these malts add their own sugars, flavors and aromas to a beer, their primary contribution is to body or mouthfeel. Without dextrin malts, some beers would taste too thin for their flavors.
As you can see, Barley alone can create a huge variety of beers. Barley varieties and maltsters can create variations even in the same category. For this reason, brewers often use the same suppliers for their malt, as a variation in supplies is often a variation in the finished product.
Some specific varieties of barley are even named so that brewers may find the barley variety even after it has been malted. Golden Promise is an example of this. This barley hails from Scotland and the UK and has a light malty flavor, with a good grainy sweetness. It differs enough from other varieties that it is worth finding for brews that require its particular characteristics.
In the next installment of Beer School, we will look at adjunct grains and sugars that add their own characters to the beer.
For more information on beer and brewing grains I highly recommend
How to Brew by John Palmer
Radical Brewing by Randy Mosher
and Tasting Beer by Randy Mosher