I hate the fact that I have been reading through several different postings for the last few hours and still feeling like I am missing the basics.

I joined United Mileage Plus 3 months ago and only have about 20K miles. I should reach 25K in the next two weeks. But, I am trying to find out how to book the "upgradeable" booking class of tickets. I seem only to be able to find booking classes like U or H, no Y or B. I would like to try and get upgrades and take advantage of any opportunities for additional EQM's whenever possible. What are some of the different ways to get the best seats?

Also, is there any reference material for all the acronyms used on here?