I want to start with malt because the malt provides the raw materials for all the wonderful biochemical processes that occur during brewing and that result in alcohol, of course, but also all the other flavors that form the body of the beer. But to get to malt we first have to go back to grain.
Grain seed contains the genetic information to grow another plant. But to have any hope of achieving that aim, it also has to have a large store of energy to get the whole process started. Until the first leaves emerge, and the plant can start capturing the energy from the sun, it has to completely rely on what it brought with it. The energy is packaged as starch. Starch is comprised of long, branched chains of sugar molecules, so it is a space-efficient way to stow the cargo, but the chains have to be broken before that energy can be accessed. To bust open the chains, the grain also has a supply of enzymes, that can snip apart the starch into its constituent sugars molecules. When the grain is malted, those enzymes are harnessed to do that exact job: convert the starch into sugars than can be fermented by the yeast into alcohol.
Malting, therefore, is basically playing a trick on the grain. By warming it up in a moist environment, the grain is fooled into starting the germination process. The enzymes are activated in those favorable conditions, and they begin to transform the store of starch into useable sugars. Then, before the grain can get as far as sprouting, the process is abruptly halted by drying, and we are left with malt.
As you will discover with all aspects of brewing, the theory is relatively simple, then you walk in to a homebrew store and see a wall of shelves filled with a massive variety of malts and you realize the many layers of complexity built on top of the basic concept. So lets think about some of the factors that can alter the properties of the malt.
First of all, consider the type of starting grain. Barley is the most popular malting grain for making beer and whiskey (the two drinks are closely related). The reason is that the starches in barley are readily converted to sugar. In other words, they are relatively easy to malt. But other grains are used, either on their own on in conjunction with barley to add different characteristics to the beer. Common examples are wheat, corn, rice and oats.
A second way the malt can be altered is by roasting it. This is a delicate process since we do not want to completely denature the enzymes that we will later rely on to perform additional conversions. But different roasting temperatures and times can provide different colors and tastes. As the malt darkens, nutty, chocolatey, earthy or even smoky flavors begin to be produced.
So now we know that malt is modified grain that provides the sugar for fermentation and contributes to the flavor and color of the beer.