What are growing pains exactly?

See below: Growing pain refers to limb pain of unknown etiology, affecting children aged 3 to 10. Child typically complains pain over legs only at night. Limb pain extending to following day or associated with limping or joint swelling certainly requires proper medical attention.

Helpful link: A helpful link with good overview of growing pains: http://kidshealth.org/parent/general/aches/growing_pains.html#