Originally Posted by
BA or bust
The answer you want is in the 'Ultimate Guide' sticky.
The trouble is that it isn't really. For example, four fare buckets are described simply as "Discount Club" with no indication of the difference between them. Of course things like the level of flexibility depend on the full fare code, not just the first letter, but some generalizations and further comments can be made. The sticky doesn't even rank them from most to least expensive.
And there are eight called just "Discount Economy".