Great question, found a link that may help you:
http://www.dimensionsguide.com/drum-stick-size-chart/
In general, smaller numbers indicate thicker sticks, and the letter B usually indicates a thicker stick than the letter A.
So a 5A is thicker than a 7A, and a 5B is thicker than a 5A. However, there are exceptions too. For example, a Vic Firth 1A is thinner than a Vic Firth 2B, even though it has a smaller number.
Also, there's no universally accepted standard between the stick manufacturers about exactly how big a stick with a particular number/letter combination is.
In general, the web sites of the different stick manufacturers usually have exact measurements of every stick they make. For example, check out Vic Firth: http://www.vicfirth.com/products/americanclassic.php
Man - check here: http://www.vicfirth.com/products/americanclassic.php
back in ancient days when i first started playing there were pretty much three stick designations:
A (as in a 7A) was an "orchestra" model
B (as in a 2B) was a "band" model, such as a concert band
and
S (as in a 3S) was a "street" model, such as for marching bands.
now with so many sticks and manufacturers, it's a moot point for the most part.
D?...................??????????????
As stated, the letters have pretty much become redundant. I've only ever seen the letters listed as per Nickg's post (A = orchestral, B = band, S = street). From memory, Vic Firth is the only company that make an 8D model, yeah? So I'm guessing there is little correlation with anything other than a marketing perspective.
In this day and age when a 7A from one stick company is remarkably different to the 7A model of another company (in both length and diameter), it's better to ignore it all for any purpose other than being able to find the sticks you like.
Given there is little uniformity between one company and another, don't get too bogged down trying to make sense of it all.....you'll be there for years and still end up none the wiser. Try several pairs and let your hands tell you what they like.