Traditionally, a station wagon is a stretched sedan body with a rear hatch. This means that it
should be significantly longer than other body styles, with more rear cargo capacity than a simple hatchback. However, this isn't always the case.
I would say that these days, a sedan-based hatchback (like the WRX or Dodge Magnum) is usually a wagon, while coupe-based hatches (ZX3, Civic) are hatchbacks. Then you get oddities like the ZX5, which is a coupe-based-sedan-based-hatchback.