Striae distensae, a common skin condition, do not cause any significant medical problem; however, striae can be of significant distress to those affected. They represent linear dermal scars accompanied by epidermal atrophy.
See the image below.
Striae distensae affect skin that is subjected to continuous and progressive stretching; increased stress is placed on the connective tissue due to increased size of the various parts of the body. It occurs on the abdomen and the breasts of pregnant women, on the shoulders of body builders, in adolescents undergoing their growth spurt, and in individuals who are overweight.
Factors leading to the development of striae have not been fully elucidated. Striae distensae are a reflection of "breaks" in the connective tissue. Skin distension may lead to excessive mast cell degranulation with subsequent damage of collagen and elastin.  Prolonged use of oral or topical corticosteroids or Cushing syndrome (increased adrenal cortical activity) leads to the development of striae. Genetic factors could certainly play a role, although this is not fully understood.
In a letter to the editor of the Journal of Investigative Dermatology, Tung et al conducted genome-wide association analysis and found evidence that implicates elastic microfibrils in the development of nonsyndromic striae distensae. 
Approximately 90% of pregnant women, 70% of adolescent females, and 40% of adolescent males (many of whom participate in sports) have stretch marks.
Stretch marks affect persons of all races.
Striae affect women more commonly than men.
Stretch marks affect adolescents, pregnant women, and patients with excessive adrenal cortical activity.
Striae distensae are usually a cosmetic problem; however, if extensive, they may tear and ulcerate when an accident or excessive stretching occurs.
Adolescents with striae can expect their striae to be less visible with time.
Treatment with tretinoin, flashlamp pulsed dye laser, and chemical peels significantly improves the clinical appearance of early, active stretch marks.