"Shin splints" is a laymens term for medial tibial stress syndrome (MTSS).
MTSS can be a very painful, disabling condition.
MTSS is generaly an overuse injury when the muscle pulls on the bone through the periosteum (connective tissue). When muscles contract, they pull on the tendons which in turn pulls on this periosteum making it inflammed and painful. This can happen with overuse, poor biomechanics (rolling feet in) or excessive muscles tightness.
Pain is usually felt along the inside border of the tibia (shin bone). It can be painful as you start exercising and pain may decrease as you warm up, however excessive strain of this over long period of time may mean the pain is present most of the time.
If you suspect you have MTSS, you should see your sports medicine professional or podiatrist. The condition will not resolve untreated. Prolonged strain without appropriate treatment can sometimes result in stress fractures of the tibia. A few common options with treatment include: