is a link to an article about DOMS. Some relevant passages from it are written below:
For those starting a new exercise routine or learning a new sport, some muscle soreness may be felt from 12-48 hours following the activity. You may also experience muscle stiffness, fatigue and weakness. Rest assured, this is a normal response to unusual exertion as the muscles adapt to the new stress. Over time this adaptation leads to greater muscle strength and endurance and the same activity will no longer result in soreness. Delayed onset muscle soreness is generally the worst within the first 2 days following the activity and subsides over the next few days.
It is thought that delayed onset muscle soreness is the result of microscopic tearing of the muscle fibers.
The amount of tearing (and soreness) depends on the activity, the intensity and the time of the activity. Any movement you aren't used to can lead to DOMS, but eccentric muscle contraction (movements that cause muscle to contract while it lengthens) seem to cause the most soreness. Examples of eccentric muscle contractions include going down stairs, running downhill, lowering weights and the downward motion of squats and push-ups. In addition to small muscle tears there can be associated swelling in a muscle which may contribute to soreness.