A. The soreness you feel within the muscle is referred to as Delayed Onset Muscle Soreness (DOMS). When we place a stress on our muscular system, it can cause tiny tears in the tissue. This is actually a good thing. When we create microscopic tears in the muscle fiber, it causes the body to respond by healing and building them up to be stronger. However, over time the muscles adapt to the stimulus and the growth process is minimal.
It is important to vary your workout in order to create a new stress to stimulate growth. This is true not only for muscles but most aspects of life. It is during the stressful, uncomfortable periods of life we grow and become stronger. But, I digress... back to the question... Changing your workout routine periodically will allow for a varied stimulus and more progress. Stimulus can be changed by making simple changes such as increasing the weight, reps, order of exercises, amount of rest between sets or completely changing the workout by switching from free weights to TRX or crossfit type workouts.
An important point to remember though, just because you weren't sore the next day, doesn't mean you had an ineffective workout! I personally have specific muscle groups that no matter how hard I train them, I never get sore. I can literally curl the 115 lb dumbbells and my biceps won't be sore the next day! But, if I just think about doing an incline dumbbell fly, my chest is sore for three days! The biceps were still being effectively trained. Don't gauge your workouts only by muscle soreness...