Groin strain seems to be one of those maladies that's always affecting professional football players. My Dad used to joke about it by asking what exactly it was they were doing to strain their groin in the first place. This is probably why I grew up thinking that it had nothing to do with exercise, or at least exercise on a football field.
Groin strain is pain and tenderness over the groin, which is the hollow between the lower abdomen and the thigh. The pain usually follows overstretching of the muscle due to running or similar sporting activity. The two muscles commonly affected are the adductors, which are on the inside of the thigh and pull it up and rotate it inward; and the rectus femoris at the front of the thigh that also help to flex the limb.
Groin strain can be mistaken for osteoarthritis of the hip and lower spine, osteitis pubis (which is a inflammation of the pubic bones), or an inguinal hernia.
It is very difficult to treat, which is why very expensive football players only end up playing one match in a season, never to be seen again (except out drinking with their buddies, with beautiful girls hanging off their arms). If it's just a muscle strain, then physiotherapy will improve the pain. However, some groin strains are so severe that a small piece of bone is pulled away from the hip by the muscle, which then has to be reattached surgically.