Welcome to FT! At the top of the United forum you'll find a sticky thread called United FAQ. It's full of good information... within that thread there is a link to a "booking code" thread that has information about all the United booking codes.

Generally speaking... Y/B/M/E/U economy booking codes are (usually) refundable and changeable (Y&B also get you a 50% EQM bonus) and H,Q,V,W,S,T,K,L are (usually) non-refundable and subject to change fees. Rules change frequently of course.