In the case of the "B" at the end of the 4000-series CMOS chips, that is to distinguish it from a 4000 or 4000A. The "B" series is a more rugged part, less prone to ESD because of some built-in protection. I've never blown a "B" version; a lot of the earlier CMOS chips have died under even my most careful handling.
Manufacturers do differ quite a bit, but as already mentioned, the letter preceeding the "part number" is usually manufacturer specific, although there have been gobs of changes there over the past 20 years. SN tended to be Texas Instruments; Motorola used M and µ a lot; "f" for Fairchild before it was bought by National before National was bought by Texas Instruments. AD preceeds the bulk of Analog Devices chips, LT for Linear Technology. The list goes on and on and there are zillions of exceptions.
The last letters are very manufacturer specific. Also as mentioned previously, some can imply temperature ranges, accuracy or specifications improvements. Most of the various letters are package designations, whether referring to the various sizes of DIP, "canned" and the plethora of SMT packages. I've seen letters stream out to five or more characters after the main part number on some analog chips from Maxim.