The Oktoberfest is an enormous beer festival that originates in Munich, Bavaria, Germany. The first Oktoberfest was a celebration of Prince Ludwig of Bavaria and Princess Therese in 1810. Nowadays Oktoberfest is celebrated worldwide, seemingly wherever Bavarians hungry for beer have settled.
From what I have gathered so far it involves fairground rides, dressing in traditional clothing and drinking large amounts of GermanBeer which I highly reccomend. The beer, called Märzen, is specially brewed for the event, is even stronger and darker than normal GermanBeer and is served in 1 Litre glasses. This is served only to those who have found a place at a table in on of the specially erected tents, called Bierzelte, with long tables each tent seats up to 1000 people, it is however unsuprising that often there are no seats available after 10:00am when you learn that last year (2002) over 6 million people attended the Oktoberfest in Munich.
The Oktoberfest lasts for two weeks starting between September 16th and 22nd and carrying on until between October 1st and the 5th of October.
A good many folk wear traditional dress at the Oktoberfest, for blokes this is lederhosen for women it is dirndl.
Dirndl is hot.
Oktoberfest beer is neither strong nor dark, so far as I have seen. It's just about as dark as a pilsner, but a hell of a lot sweeter in taste. It's ok, but not great. There are many German beers, year-round, that are a hell of a lot better.
In addition, Oktoberfest is strictly a Munich thing. Don't go to Berlin or Heidelberg expecting to revel in their Oktoberfest activities. Also, lots of Muenchners don't even go to the Oktoberfest since it's fairly well touristy. Just because there are lots of German tourists there doesn't mean that it's any less touristy.