Those are fare booking codes, which really don't have that much to do with seating. Just a quick overview:

There are up to 3 classes on United. First, Business, and Economy. Within each class, they sell a certain inventory of seats under the various booking codes.

In general, First Class includes booking codes F (full fare First Class), A, and P(discounted). Business Class includes C (full) and D (discount). Economy is where it gets fun, with Y (full full), B (sort of full), M, H (discount), Q, V, W (more discounted).

There are others, including X*, and N* for mileage plus related booking inventories, and other special discounts... K, L, G, etc...

Within each seating class, generally as you go "down the list" the fares get cheaper and your ticket has more restrictions. Before the advent of Economy Plus, buying a ticket out of a more expensive inventory didn't get you a better seat within the seating class. But now, full fare tix can get an Economy Plus seat assignment. (Has anyone determined if this only applies to Y or also B?)

So: the long and the short of it is this: a M ticket, though more expensive than a Q ticket, will not get you a better seat. But if you need to make changes to that ticket, the M ticket will probably be more forgiving than the Q...

Hope I didn't confuse you too much.