The following chart gives some of the letters used to designate each
class of service. Note that Fn means Night/Offpeak Coach in the First
Class compartment, and Yn means Night/Offpeak Coach in other than the
First Class compartment.

                        Regular Premium Discounted      Night/Offpeak
First Class             F       P       A               Fn (Coach in FC seat)
Business Class          C       J       D               Cn
Standard                S       W                       Bn
Coach Economy           Y               B, H, M, Q, T   Qn, Yn
Thrift                  K               L, V            Vn, Kn
Supersonic              R
Shuttle Service         U (No reservation needed, Seat guaranteed)
Shuttle Service         E (No reservation allowed)

In reality there is no difference between classes F and P, nor between
classes C and J. The Q class is usually used for discounted student fares.

If you want the cheapest fares, look at the Thrift and Coach Economy
classes. 

Airlines have started eliminating First Class, because many companies
now have policies that won't let their employees claim a first class
ticket on their travel expenses.  Instead, the airlines have started
upgrading their Business Class as a marketing ploy (and, of course,
company policies are now requiring employees to travel coach).

Some parts © 2009 Advameg, Inc.