Sarcy -
yes, in a nutshell, quality miles are the ones where you're running harder than your normal training pace and include any combination of the following: repeats, intervals, pyramids, hill reps, fartlek, tempo runs, tempo intervals, and so on. as Allan say, even strides can turn a slow run into a quality work-out. the variations are as numerous as there are running experts.
you need a sensible amount of these every week if you want to get faster. which you do, depends on your goals (and also on what phase of your training cycle you are in). it's not advisable though to be doing more than one hard work-out a week (e.g. intervals) if you're starting out. the formula you mention is not bad to begin with, but once you're used to speedwork you can start adding to it, e.g. with a fartlek session which isn't going to wear you down as much as a tempo run or intervals. again there's all sorts of variants, depending on what you're trying to achieve.