The word ‘listing’ is a short-hand term used to describe one of a number of legal procedures which help English Heritage to protect the best of the country’s architectural heritage. When buildings are listed they are placed on statutory lists of buildings of ‘special architectural or historic interest’ compiled by the Secretary of State for Culture, Media and Sport under the Planning (Listed Buildings and Conservation Areas) Act 1990, on advice from English Heritage.

A listed building is a double-edged sword. They are typically more desirable than non-listed buildings, however more stringent planning rules apply to them, so you need to be much more careful about the types of work that you carry out on them, and also the materials that you use.

Listed buildings are graded to show their relative importance:

Grade I buildings are those of exceptional interest
Grade II* are particularly important buildings of more than special interest
Grade II are of special interest, warranting every effort to preserve them
There are 370,000 or so list entries currently protected by listing, and of those by far the majority – over 92% – are Grade II. Grade I and II* buildings may be eligible for English Heritage grants for urgent major repairs.

For more information, have a look at the following websites:
http://www.english-heritage.org.uk
http://www.spab.org.uk