A cross between a caravan and a trailer-tent.
Basically a folding camper is like the bottom half of a caravan, with a canvas top-half,
which folds out from the body.
The larger units, like the Pennine
have beds at front and back which fold out lengthways, so that you have one bed over the tow-hitch, and one over the rear of the unit.
The big advantage compared to a caravan is that the beds do not need to be folded away to create living space.
Yes, I know some caravans also have permanent beds, but a lot of caravans do not.
While towing, the folded unit is much easier to tow than a caravan, as the height of the folded unit is usually less than the height of the car.
Although you cannot usually see back over the top of the unit, it is not so daunting as towing a caravan, and has much less wind resistance.
The most common types seen in the market in the UK and Ireland are Pennine and Conway.
Both of these companies are now part of the one group, and are very very similar.
The main difference is the method of unfolding... The Penine units have beds which fold out on hinges,
whereas the Conway units have beds which slide out.
For more info on these products, see the penine website at The Pennine Group
A very good UK based forum for Folding Campers, and also Caravans and all camping info is