While a few codes are widely standardized, such as Y for full-fare economy, others vary from one airline to another. ExpertFlyer has complete lists for all but some minor airlines, including the codes for upgrades and award tickets that cannot normally be booked directly. They also offer a five-day free trial, so if all you want to do is check the code lists, it needn't cost you anything. (Most frequent travelers who have taken them up on the free trial have been hooked, so beware - you have been warned!)