defined: represents an excessive proliferation of collagen [scar tissue] following trauma to the skin.
Clinically, a keloid apperars as an elevated, firm protuberant nodule  [ Principles of Dermatology by Lookingbill & Marks]  -- it is an overgrown scar!

epidermis maybe atrophic, dermis has highly compacted whorls of collagen that push up.
incidence is higher in those aaged 10 - 30 yrs.  Blacks are more prone to keloids, with those in black African populations having a prevalence of about 6% [ 1986 statistic]
can occur anywhere but tend to appear on upper back, chest , earlobes and deltoid area
keloids develop over weeks to months following trauma [even ear piercing can cause keloids!], surgical wounds, or other wounds, and may be seen as a consequence of severe acne.

surgical removal is not advised, as the keloid often grows back, larger than before.
 

resources used:
 Principles of Dermatology by Lookingbill & Marks.  Saunders Publishers  1986
The Merck Manual 15th ed,  Berkow & Fletcher editors.  Merck Sharp & Dohme Research Labs 1987