DOMS is Delayed Onset Muscle Soreness. The ache you get the day after an effective workout. You over-stress the muscle, and break it down, and those tears in the muscle fiber need to be rebuilt stronger. Till that happens, you are sore.
If you are not over-stressing the muscle, you won't get sore, but it is hard for the muscle to grow, since the muscle increasing size is an adaptation to being overused, and being forced to grow to be able to handle the new stress. Bigger muscles can handle more weight, reps, sets etc. If their is no excessive stress, then your muscles do not need to grow. For me, it was extra sets that I wasn't used to. I'll probably stop hurting by tomorrow, and don't do legs again till Wednesday.