Scars are fibrous tissue that forms on skin in the process of healing. Any trauma to the skin like infection, cuts or bites can cause scarring. How a wound heals can produce hypertrophic , keloid or atrophic scars. Scar tissue is different from normal skin because of the particular formation of collagen during the healing process and it tends to be less elastic and functional like normal skin.
Stretch marks are formed during quick expansion of skin during puberty surge or pregnancy. The elastic fibres in the dermis starts to split when the growth expansion exceeds its stretching capacity. Excess corticosteroid can also cause stretch marks to appear. They are seen as red, pink, purplish or white lines just beneath the skin surface and is usually permanent.