Not Medical Advice: Hives (urticaria) are red, and usually extremely itchy, raised areas of various shapes and sizes that appear on the skin and are produced by histamine and other compounds that are released from mast cells (normally found in the skin).
Histamine causes leakage of fluid from local blood vessels, which leads to swelling in the skin. Antihistamines (such as Benadryl) are the most common treatment for relief of symptoms like itching and redness; the condition of having hives itself normally goes away on its own.
Hives are extremely common and sometimes caused by allergies (foods, medications, insect stings or bites, etc.). Hives can rapidly change size and move around, sometimes disappearing in one skin area and reappearing in another, sometimes within a matter of hours.