I'm used to seeing J for business.

F, J, Y are what are normally called "full-fare" first, business, and coach, respectively. They are unrestricted refundable tickets, and they cost the most. Full-fare Y often has other perks, like business check-in, complimentary upgrade to J, or other such, depending on airline.

Other booking codes, like Z, X, etc. are for various classes within first, business, and coach. These represent restrictions on the ticket, such as change fee applicability, date restrictions, or even mileage earning applicability at the lower economy fares. Naturally, these tickets are cheaper than full fare in the respective class. They are also limited in capacity; the airlines like holding back seats to sell at full Y, since they get more money that way. But not everybody pays for full fare, after all.

That's about the gist of it.