How many styles of beer are there?
There are only two types of beer: ale and lager. There are many “styles” of each.
Ale is fermented using Saccharomyces Cerevisiae and fermentation is very active, resulting in a thick, dense foam that appears to be boiling when fully active. The resulting impression is that is “top fermenting” which is only partially true. The cap of froth covers a liquid that is opaque from the yeast as it ferments the sugars into alcohol and carbon dioxide with a great deal of enthusiasm.
This yeast is the oldest and most varied strain developed by brewers. Each variety of ale has its’ own strain of yeast that provides particular aromatic and taste profiles.
Lager is fermented using Saccharomyces Carlsbergensis, syn. of Saccharomyces Pastorianus. (See: yeast)
Both of these types are the home of a number of styles. The determination of what constitutes a "style" has been generally codified by brewers and organizations such as the Association of Brewers, The Beer Judge Certification Program, CAMRA, and organizers of the World Beer Cup (not to mention beer writers who never miss an opportunity to argue what makes a style legitimate).
The essential difference is the content of the brew. As each chateau has a "signature" that wine connoisseurs can readily detect, certain geographical areas also provide "signature" brews that are just as distinct. The basis for different styles is usually found in the basic ingredients of the brew that are indigenous to a specific area.
It is important that you know the "real thing" when you taste it. It should be similar to the sensual experience of first tasting genuine Roquefort Bleu cheese, or a ripe tomato fresh off the vine. The Roquefort put all other blue cheeses in context, and that just-picked tomato gave you a different perspective on all those gas-ripened hothouse supermarket tomatoes you once paid way too much for.
The following is not a list of every style of beer known to brewers. I chose these styles because they offer a sufficiently broad range of taste experiences to introduce you to the "essential" styles of beer. I do hope you become intrigued enough to search for even more esoteric "styles". I can guarantee it will prove to be quite an adventure.