Wound healing

When person is wounded, the tissue at the wound is damaged. Many organisms can replace this damaged tissue with new tissue. This occurs in several steps, and is generally known as tissue healing, or wound healing. 
In undamaged skin, the epidermis (surface layer) and dermis (deeper layer) form a protective barrier against the external environment. When the barrier is broken, biochemical events repair the damage. This process is divided into predictable phases: blood clotting (hemostasis), inflammation, tissue growth (proliferation), and tissue remodelling (maturation). Blood clotting may be seen as part of the inflammation stage instead of a separate stage.
In general, after the wound is fully healed, a scar will remain. Even after the scar has appeared remodelling processes take place. It can take a very long time until this scar has reached its final form.
Sometimes complications from wound healing can occur. A common one is infections during the healing process, which will lead to more scars or larger ones. 
This process is not limited to humans, but it does not apply to organisms with a hard exoskeleton such as arthropods.