An Area of Outstanding Natural Beauty, which for the sake of convenience is often referred to as an AONB, is a designation given to specific areas of England or Wales which are of national importance due to the beauty of their landscape and scenery.
They are a bit like National Parks, with the exception that whereas each National Park has a National Park Authority to run the Park, but AONBs have no such layer of adminstration. The most that they have is a manager employed by the local council, which is usually allocated some extra money from central government in order to promote good management and sustainable development within the AONB. The main thing about an AONB is that the same level of planning restrictions that apply to National Parks also apply to AONBs, so that the enviroment is protected from any unsympathetic development.
The Countryside Agency has the job of designating AONBs for England and the Countryside Council for Wales performs the same function for Wales. Scotland, as usual, does its own thing and instead of an AONB has the different concept of the National Scenic Area, which are designated by Scottish Natural Heritage.
There thirty-six Areas of Outstanding Natural Beauty in England, four in Wales and one, the Wye Valley in both. The smallest is the Isles of Scilly which is a mere 16 square kilometres (or 6 square miles) and the largest is the Cotswolds, which totals some 2,038 square kilometres (or 787 square miles).
In both England and Wales
Sourced from http://www.countryside.gov.uk, http://www.ccw.gov.uk and http://www.naturenet.net/status/aonb.html