Nigel,
I can only give you my opinion from what I know and I don't know if this suits the dictionary definition or if there even is one. I'm sure if I'm way off I will quickly corrected. Heathland golf is most commonly associated with the UK (although germany, france and The Netherlands to name a few also have some good ones). While I've not personally experienced any of them in the US, the closest course in terms of playing condition I can point out for you would be perhaps the likes of Bandon Trails (ex the heather). However the ground contours can be similar Bandon Trails is more hilly and rugged of course than any heathland I've played.
A heathland course for me is kind of in between a links course and a parkland course. It has many characteristics of links golf, for example, sand based terrain, hard, fast conditions unpredictable bounces due to hobbles and knobs in the terrain (although not always), often with heather and even gorse playing a roll. Heathland courses also have trees although I believe in most cases not as many as parkland courses. They are always inland.
I'd also add that my view is tainted because most of the heathland courses I've played benefit from solid old school famous architects like Harry S. Colt.
Parkland is most commonly associated with US golf. Lush green fairways, target golf, forest courses, perfectly manicured like taking a walk in a park. Fairways are soft and lush (not all the time of course but I've rarely played any that were not). The elements seem to be much less a factor and more often than not you get what you expect, so you are much less likely to get the unexpected bounce. Many parkland courses are manufactured, meaning only there has been significant earth movement. (I'm certain there are exception before anything gets offended here. It is in my experience more common for links golf and heathland golf for that matter to use the natural lay of the terrain.
Please feel free to correct me or add to this as you see fit guys.