Munich's Oktoberfest began not as a beer festival, but with a royal wedding—on October 12, 1810, Crown Prince Ludwig I of Bavaria married Therese of Saxe-Hildburghausen, and Bavaria rejoiced—after all, he would not be forced to abdicate for another 38 years (after an affair with renowned Irish actress/dancer/courtesan Lola Montez)—and everyone in Munich was invited by the Bavarian National Guard to enjoy the five-day party. The field in which most events were held became known as Theresienwiese, in honor of the princess. In fact, it was so much fun (and remuneratively rewarding for Munich's city fathers) that it was decided to celebrate the royal couple's anniversary each year in similar style.
The initial draw for the annual festivities was not the beer, but the horse racing—in fact, food and drink stalls did not become standard practice until 1818. Things did not really become organized until the late nineteenth century, when some of the other instantly familiar elements of Munich's Oktoberfest arrived to the party—wearing traditional lederhosen and dirndls began in 1887. While these other changes were taking place, beer (which had been served from informal tents and stalls for many years, occasionally as a highlight, but often as just part of the scenery) began to move to the fore.
Beginning in 1896, Munich's major breweries began officially sponsoring semi-permanent beer halls; only beer brewed within the city limits was allowed to be served. Those six breweries—Augustiner, Hacker-Pschorr, Hofbräu, Löwenbräu, Paulaner and Spaten—are still the anchors of the festival, and each tent maintains its own style to this day. But the beer served in those tents has changed over time, though the style has remained nominally the same.
What is now widely known as 'Oktoberfest' beer has long been known as Märzen—so called because it was traditionally brewed by March. The lager was the subject of a Brauordnung—a Bavarian brewing ordinance issued in 1539 that restricted its brewing to the period between St. Michael's and St. George's day (late September through mid-April). The beer would be kept cool in caves and cellars and consumed throughout the summer (when it was too warm to brew effectively); by the nineteenth century, choice bottles were being sent on to Oktoberfest. Traditionally, Bavarian brewers used darker malts for their lagers, so the 'orignial' Märzen was probably a darker, maltier beer—but things began to change in the mid-nineteenth century.
Beginning in 1841, Gabriel Sedlmayr and Anton Dreher began to experiment with lighter malts and different yeasts; the resulting lighter-colored beer became associated with Dreher's home base of Vienna, while Sedlmayr (back in Munich, where he owned Spaten —his initials remain part of the company logo today) continued to call the modified beer a Märzen, albeit one 'gebraut nach Wiener Art" (brewed the Vienna way). The Vienna version of the beer also spread far and wide—notably to Mexico—but that's another story. Spaten continued to tinker with the recipe; Gabriel's son, Josef Sedlmayr, won acclaim at the 1872 Oktoberfest with a version that was a bit darker than a Vienna Lager, but lighter than the older-style Märzen; the beer was the first specifically labeled as an 'Oktoberfestbier.' The toasty, reddish-amber lager became the definition of the style, and it would remain so for decades.
But with the advent of better refrigeration (and a trend toward clear glass over earthenware steins), many European lagers became lighter and lighter in color as those styles became more popular. By the time Oktoberfest became the enormous international event it is today—attracting over six million visitors each year—the pale lager (usually something closer to a Helles or Dortmunder Export style) was the sort most often found in the beer halls. This may have come as a surprise to American visitors, as most American-brewed Oktoberfest beers tend to be closer to the traditional style—typically an amber or reddish lager. The 'big six' German breweries continue to export darker Märzens to the US (although the lighter versions are often available as well, in some cases), and the Brewers' Association and other industry organizations recognize the divergence within the style. Not only do the beers look (and taste) different, but the American version often packs a more powerful kick; the beers served in liter tankards in Munich tend to be lower in alcohol.
Of course, while Munich's Oktoberfest celebrations are held on the original site, they are hardly the only such autumnal festival. Blumenau in Brazil is said to be the second-largest Oktoberfest in the world, while Canada is not far behind with the Kitchener-Waterloo Oktoberfest; even World of Warcraft has an Oktoberfest equivalent. The beer may have changed over time, but it's still an ideal excuse for a celebration—prost!