The pain that is associated with exercise is refered to as Delayed Onset Muscle Soreness or DOMS. DOMS is the same thing as exertional rhabdomyolysis. While mild DOMS is common, severe rhabdo is an uncommon diagnosis. But because its complications can be severe, personal trainers as well as medical clinicians need a thorough understanding of this syndrome. When skeletal muscle cell membranes are damaged, their intracellular contents enter the bloodstream and can cause potentially serious sequelae, even death. Intense exercise, some viral infections, and certain genetic disorders increase the risk. Serum creatine kinase levels are the diagnostic gold standard. The treatment of rhabdomyolysis consists of early detection, therapy for the underlying cause, measures to prevent renal failure, and correction of metabolic complications.
Most of the time DOMS is not dangerous and will clear up on it's own in a few days. It is NOT related to lactic acid as that clears out of one's system in about 90 minutes. You can help lessen the severity of DOMS by using certain restorative techniques such as branched chain amino acids immidiately after exercise and ice massage. Of course, the best method is to
gradually increase one's workload over time.
Here are a couple of links...
http://www.nlm.nih.gov/medlineplus/ency/article/000473.htm
http://www.med.harvard.edu/JPNM/BoneTF/Case18/WriteUp18.html