Beer is the name of a large family of alcoholic beverages that result from the fermentation of grains. Beer is one of the oldest alcoholic beverages, and the third most consumed drink on the planet, behind only water and tea. Beer probably began around 9,500 years BCE, when cultivation of grains began. Chemical traces of grain fermentation date to around 3,500 years BCE. By 3,000 BCE, Celtic and Germanic tribes had spread brewing knowledge throughout most of Europe, though what they drank had little in common with what we know today as beer.
With the rise of monasteries after the fall of the Roman Empire, brewing became standardized, as monks were heavy consumers of beer. Hops as a flavoring agent became known in the 9th century CE, and became part of the 1516 Reinheitsgebot (purity order), the Bavarian law that dictated that only water, barley and hops could be used to make beer. This law was intended to protect food crops like wheat and rye from being diverted into beer production, and the penalty for breaking the law was uncompensated confiscation of illegal barrels. Note that the law says nothing about yeast, which were not discovered as the primary driver of fermentation for nearly 350 more years.
Beer is mostly water, and as such, good sources of high mineral content water were vital to beer brewing. Many styles of beer are intertwined with a particular source of water, such as Stout beers needing hard water, or Pale Ales requiring high levels of gypsum to achieve aroma and flavor. The grain bill can vary widely, with some beers being 100% malted barley, which is partially sprouted, then roasted to different levels from very light to almost the color of coffee, with darker roasts giving more caramelized flavors. Almost any grain can be used to make beer, with some other common sources being wheat, rye, oats, corn or rice, though corn and rice are looked down upon, as they're cheap, relatively flavorless additions of fermentable sugars. Beers which rely on other sources of grain than what is required are called adjunct beers.
The brewing process is fairly simple: Malted grains are mixed with hot water and allowed to hydrate for a few hours. Enzymes from the barley convert starches to sugar, which dissolve into the water. This liquid, known as wort is drained off, and the grains are rinsed to capture as much fermentable sugars as possible. The wort is put into a kettle (known as a copper) and cooked for a few hours. This drives off excess water and sterilizes the wort, preparing it for the addition of hops. Hops add bitterness and citrus, piney or floral aromas and flavors, but they also are antibiotic, preserving the wort for the addition of brewer's yeast. The longer the hops stay in the wort the more bitterness they impart, but less hop flavor. Bitterness in beer is measured in International Bitterness Units, or IBU's. Color in beer is measured in degrees Lovibond, named after Joseph Lovibond. The amount of fermentable sugars in the wort is measured in degrees Plato.
The flavored wort is then cooled and yeast is added. Most beers are fermented by Saccharomyces cerevisiae. Here, the beer splits into two distinctive styles, depending on what kind of yeast is used. Ales are top-fermented at warmer temperatures, and lead to beers that are lighter and more complex, with fruity aromas and flavors. Ale fermentation is typically quick, taking a few weeks, then the beer is aged at between 40 and 55 degrees Fahrenheit for a few weeks. Lager beers (from the German Lagern, to store) have slower fermentations. The yeast ferments at cooler temperatures at the bottom of the fermentation tanks, and once primary fermentation is over, aging takes place at just above freezing for a few months. Lager beers were developed in Germany probably accidentally, when barrels of beer were cellared in cold caves over the winter months and opened in springtime. Brewers found that beers that underwent this process were cleaner and more stable than beers that did not.
After the aging process, the beer is filtered and readied for bottling, which may be in kegs, cans, or bottles. Some beers undergo a second fermentation in their receptacle, which provides a source of natural carbon dioxide as a pressurant for the beer.
In the Ale family, some sub-styles include Pale Ale and its hoppy cousin India Pale Ale, Porter, Stout, Hefeweizen (where wheat is the primary grain in the bill) and Barleywine, which is quite malty and high in alcohol. In the Lager family are Bock, Marzen, Dunkel, and Pilsner beers (yes, there's a town called Plzen in the Czech Republic, where the style was invented in the 1840's).
In Belgium, a few interesting styles of beer developed. First, there is the lambic family, which are spontaneously fermented, and have unmalted wheat and aged hops that have lost most of their flavor. Lambics are intensely sour, and depending on what ferments them (it can be a number of things, from the Brettanomyces family to enteric bacteria, to lactobacillus and pediococcus), lambic beers can have cheesy, horsey, leathery aromas and flavors. Some lambic beers are flavored with fruit -- these are named by the fruit: Kriek (cherry), Framboise (raspberry), Peche (peach) or Cassis (blackcurrant). If they're left unflavored, words like Geuze might appear on the label, which denotes a blend of older and younger lambic beers.
Most beers are between 3 and 6% alcohol by volume, though they can be as low as 1% and, using freeze-distilling methods, as high as 55% abv. (This is the infamous BrewDog beer called The End of History). Beer should be served between 45 and 60 degrees Fahrenheit, though aroma and flavor is negatively impacted below 50 degrees. As a rule, the darker and heavier the beer, the warmer it should be served. Some brewers have developed special glassware that best exhibits their beer, with the Chimay goblet being one well known example.