I've seen one once, It's a garden or area of a garden thats been constructed for growing plants that like to grow in acidic conditions such as ericaceous plants like heathers rhododendrons and azailias. A lot of peat is used for the growing medium and even sometimes blocks of peat to form edging or terraces.
Probably not very PC now a'days as peat extraction is considered by many to have a harmful impact on the environment.
The one that I remember seeing was a hilly area with mature pine trees and rocks with heathers underplanted and a low retainig wall of peat "bricks" forming edging at the side of the path, which the heathers were allowed to scramble over.