It was originally a farmhouse ale brewed in small batches by farmer/brewers in the Flemish speaking area of northern France. By WWI, many brewers moved to bottom fermenting yeast but kept the fermentation relatively high (60F). Locals would buy the beer out of primary and allow secondary fermentation in crocks at their homes.
As a style, it is extemely variable. It is similar to the "red beer" phenomenon in that you have a great deal of leeway in what you throw into the kettle since it is not a specific style. Consider it a beer typical of this region rather than the overly specific AHA definition.
Looking at the largest selling commercial imports, they have an OG of 1060-1074. 5.6-6.6% ABV. There should be an ale like fruitiness, but restrained. The accent is on malt, usually of a spicy, aromatic specification. Vienna malt is predominant grain in the mash. A long vigorous boil is used for carmelization and the saccharificaton temperature leans towards a dextrinous wort. The color is 35-40 EBC. Hops are spicy, but soft. 22-30 IBU using Brewers' Gold, Hallertau, Spalt, and Hersbruck. The water should be soft. Some people insist on cellar characteristics of oak, cork, or ageing. Lagering at 35F for one month leads to smoothness. Commercial companies usually produce several beers of increasing strenghts and colors as well and stronger winter beers going from deep gold 4.5% ABV to 7%ABV dark winter beers.
Kit "Travels with Chiles" Anderson