A Parkway is a station, usually on a major route that serves as a point for Park & Ride rather than anything in the vicinity, although its worth pointing out that some locations such as Bristol Parkway have attracted quite interesting commercial development and it is becoming an area in its own right. In fact, some, such as Tame Bridge Parkway aren't for a certain place at all (there is no such place as "Tame Bridge").
It doesn't necessarily represent a station with a Car Park (although a Parkway must have one), and there are some in Greater Manchester and what used to be the West Midlands County which are termed "park and ride," although it's worth mentioning that Manchester and Birmingham don't have bus-based park and rides like many other towns & cities.
It's also been the case where a station does serve the town (eg: Didcot, Port Talbot) but it's also used to encourage people from smaller places nearby to drive there and take the train.
As to John_158, I wouldn't say that it's anything to do with an interchange, I struggle to think what you'd interchange for at Whittlesford Parkway (a recent renaming to a Parkway rather than a new station), Haddenham & Thame Parkway or Tiverton Parkway. Even where multiple services to different destinations & stopping patterns serve, you'd be better interchanging elsewhere.
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