Shin splints is a bad term for a problem that is common in athletes, especially runners. The term just means ‘sore leg’ so it could be due to any one of many different problems, so as a term it should no longer be used. When the term is used, it is probably most often referring to the condition of medial tibial stress syndrome.
This problem is one of a stress reaction along the edge of the shin bone or tibia in the leg. Running is the most common cause of medial tibial stress syndrome. The symptoms can usually be palpated along the edge of the bone. Initially its typically not painful when running, but eventually it progresses to the stage that it i painful when running.
Typically the problem is one of the running technique and the runner can often benefit from a gait analysis and a running retraining program to change the way they run. Typically this will involve running with a wider base of gait.