Postcode Address File (PAF)
The Postcode Address File (PAF) is a database which contains all known "Delivery Points" and postcodes in the United Kingdom. Buildings which contain several flats or businesses but only one external front door, will only have the external front door listed; the full addresses which include flat numbers, floors etc., are excluded from the PAF.
The PAF is a collection of over 28 million Royal Mail postal addresses and UK postcodes. It includes Small User Residential, Small User Organisation and Large User Organisation details. It is available in a variety of formats including Digital Audio Tape and compact disc. A charge is made for lookup services or wholesale supply of PAF data.
Errors
Royal Mail acknowledges that the PAF contains errors, and publishes forms for submitting error reports. A very small number of addresses are not listed correctly, and others (especially new developments) may not be listed at all for a period of time.
Costs and public availability
Between 2004 and 2006 a consultation was taken about the future management of the PAF. The proposal to release it for use at low or no cost was rejected, and the business model where it was used to raise money from profitable corporations was retained. The balance sheet for the PAF for 2005/6 disclosed an income of £18million, 8.6% of which was profit. Following a Government consultation, on 1 April 2010 Ordnance Survey released co-ordinate data for all Great Britain postcodes (but not their address elements) for re-use free of charge under an attribution-only license, as part of OS OpenData.
Information above is from Wikipeida, you can find more at:
http://en.wikipedia.org/wiki/Postcode_Address_File