Any beer can typically be classified as either a lager or an ale (1). A lager is any beer made with bottom-fermenting yeast and may be golden or dark in color. Conversely, an ale is any beer made with top-fermenting yeast and typically has a "fruity" flavor. Lagers are cold-brewed for longish periods of time and are typically paler, drier, and less alcohol-tasting than ales. Ales are brewed at warmer temperatures for less time, are generally higher in alcohol, and are more robust and complex than lagers.
A pilsner ("pils" for short) is golden-colored beer made with bottom-fermenting yeast (which makes it a lager). This conventional strength beer is considered by some to be a "super-premium" quality (2).
A stout is an extra-dark top-fermenting beer made with highly-roasted malts (2).
A porter is a lighter-bodied version of the stout (2).
For other types of beers, check out this website: http://www.beerhunter.com/beerstyles.html.
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Sources:
1) http://www.answerbag.com/q_view/1639
2) http://www.dougcoppock.com/dougcoppock/beer.html
