Mr Bouncy:
There are a lot more letters than that used by the airlines. As other posters have said, they don't mean anything in themselves, it's simply a way of distinguishing the different fare levels. The Virgin system as posted is a good example. In the old days revenue accounts used to scrutinise them to ensure that the correct fare had been charged for the code.
Some airlines still use F for first, C for business etc., but there are so many fare combinations out there, that most of the letters have now been used. Letters rather than numbers were originally used to avoid confusion with flight numbers, or so I was told when I was learning about these things.