Generally speaking, "hammer toe" means a bending of any of the toe joints. Joints in the middle or at the end of the toe, as well as the joint near the ball of the foot, may be involved. Toe joints can curve and buckle because of tight tendons or muscle imbalance. Some hammer toes are severe, depending on how many of the joints are involved.
Tight shoes, pantyhose or socks can be the cause of hammertoes.