Google came up with the following URL which has downloadable lists and lots of related links:
http://www.brainstorm.co.uk/uk_post_code_search.htm
(the links to the Royal Mail will probably provide the best explanations).
Postcodes are usually of the form LLnn nnLL
Although the first two LL might be only one letter, and both sets of numbers might be one digit. The furthest left bit refers to a large area - typically a city of a town in an area of countryside. The letters are chosen to "make sense". Eg. Leeds is LS, Cambridge is CB, Redhill is RH, etc.
The first set of numbers then refer to large-ish areas. A lower number is usually central. Eg. LS1 is central Leeds.
I grew up in "LS17" which is to the north east of Leeds itself and contains quite a bit of countryside.
These areas are divided further with the next number. (as far as MapPoint goes). Then the last couple of letters should in theory identify a group of about a dozen houses. Things like office builidings, colleges, might have their own individual postcodes.
I probably haven't explained that too well but hope it helps.
Swindon is a town. east of Bristol. Was built as a railway town on the old GWR from London to Bristol.
Richard