DOMS is an acronym for Delayed Onset Muscle Soreness which is the muscle soreness experienced 24-48 hours post-exercise. It can also start as early as 6 hours and last till 72 hours post-exercise.
DOMS is a common result of intense physical activity that stresses the muscle beyond what it is accustomed to. Eccentric contractions are more likely to cause DOMS because eccentric loading puts a higher demand on your muscles compared to a concentric contraction of the muscle, and with these higher loads/demands the muscles suffer microtrauma.
DOMS is thought to be microtrauma in muscle and connective tissue and causes inflammation and subsequent pain. Anyone can get DOMS and is more pronounced when a new training activity or increased training load (volume or intensity) is introduced. DOMS is not a bad thing, it is really a sign of your body trying to make adaptions in an attempt to better prepare your body to do the same activity again.