When people refer to Oktoberfest beer, they are referring to one of two things. In the first case, they refer to any beer that is brewed within the city limits of Munich, Germany. After the Club of Munich Brewers have decided to deem it as such, the beer is referred to from that point forward as an Oktoberfest Beer. The important point of distinction here is that it is a beer that is authorized to be sold at Oktoberfest, the single largest beer festival in the world (serving over seven million liters of beer in roughly 16 days).
The second reference is actually a misnomer for a beer style called Märzen. Märzen is a bavarian lager that is commonly served at Oktoberfest (hence the confusion), but all Märzens are not Oktoberfest beers, nor are all Oktoberfest beers Märzens. There is an interesting story behind Märzenbier. Back in 1536, the Bavarian government outlawed beer brewing between 23 April and 29 September because there was an increased risk of starting the countryside on fire during the warm, dry summer months. So brewers had to develop a beer style that they could make large quantities of that would keep through the summer. They came up with Märzen and would work overtime in late March and April brewing as much of the stuff as they could, then barreling it and storing it in caves to keep it cool and out of the sun. Whatever was left at the end of the summer was generally taken to Munich and served at Oktoberfest (or just drank as if the imbibers were at Oktoberfest). It is usually amber in color and has an alcohol content of 5 to 6.2% with a mild hop profile.