What do British postal codes mean?

Example: York YO24 1DP

A British postal code can have 6 or 7 letters/numbers.
The first part (general information) indicates the sorting office (eg. YO) and the postal town (24).
The second part (detailed information) contains two pieces of info. The number (1) shows the part of a bigger town or rural area, in which a particular address can be found. And the last two letters (DP) show the route taken by the postman and indicate a small group of streets for which a postman is responsible.