An automotive body style that involves extending the cab and chassis on a sedan slightly, and eliminating the trunk, so as to provide much more space for cargo and passengers. They get their name from the fact that the first wagons were used as ad-hoc taxi cabs, to haul people and their belongings from the train station to their final destination (the earliest wagons were also called "depot hacks" for this same reason).
For many years, station wagons were the biggest cars you could get. The first SUVs were actually taller, chunkier station wagons (and most are actually still designed this way). What really put a dent in wagon sales, though, was the introduction of the minivan by Chrysler in 1984. Wagon sales dropped off sharply, as more families and soccer moms found themselves buying the Dodge Caravan instead of that big ol' Country Squire or Caprice. This, combined with the SUV craze of the 1990s, put an end to the days of the old full-size wagon for good, with Ford abandoning the Country Squire and Colony Park after the 1991 model year, and the last Buick Roadmaster rolling off the line in 1996.
Small wagons, like the ones made by Subaru, managed to stay somewhat popular, and now (seeing a backlash against the SUV craze), wagons of all shapes and sizes are starting to make a comeback.