Can someone please, in as simple of terms as possible, explain to me what the different fare classes mean?

Y, YAS, S, B, M, H Q, L, V, K, G, T, U, F.... etc.

Is this information that frequent flyers are supposed to grasp? I know that "F" means first class, and "U" must mean upgrade. Do the other letters actually mean something? People throw the letters around constantly, and they must be pretty important because depending on what fare you pay for you get different things. The problem is, when you shop for a flight, it isn't immediately apparent (to me, at least) what the letter is of the fare you're getting.