What they are and how to find them.
UK postcodes are alphanumeric and between five and eight characters long (including a single space separating the outward and inward parts of the code), e.g. the code for the House of Commons is SW1A 0AA.
The 'Outward' part of the postcode denotes the postal district - for example RH for the Redhill area, and then the following number distinguishes the post town - broadly speaking the Delivery Office which services the local area. So RH1 is Redhill itself, RH10 is Crawley. With larger towns there may be more than 1 number in the outward section - Crawley includes RH10 and RH11. The 'Inward' part denotes particular parts of the town / Delivery Office area, with the first part - the number - being a sector, and the final two letters denoting a group of houses within that area.
For more see Wikipedia.