Newbie here. There's a lot of great information in different forums and I am still trying to get a handle of it. One thing that I am quite confused with is the airfare classes as I've seen that mentioned in a lot of MR deals and whatnot. I looked it up and found this:

http://en.wikipedia.org/wiki/Travel_class#Fare_class

But what is the difference between the different economy classes? I read in a couple of threads in the MR deal forums that they'd want G or K class while some don't seem to be very high on L. What does that really mean in terms of those classes? And how do you book specific class in, say, united, or whatever airline's website?

Thanks ahead