F means first class. A is also used for first class. If an airline has both, one will be for paid tickets and the other for award seats and upgrades (there may even be a third to separate those two), but it varies which is which depending on the airline.

C and J are used similarly for business class.

Most other letters are kinds of coach ticket. Y is universally the full walkup changeable coach fare. B is normally the next-highest (say, a 3-day advance). After that, it varies. (For example, U is the lowest on Delta, followed by L, and then K, which is the cheapest fare on which one can try to upgrade to business class on a domestic flight.)