What is Urticaria?
This is a reaction in the skin involving small, red swellings called wheals.
Hives occur when the body's immune system reacts to an outside stimulus, such as an allergen, in an inappropriate manner, releasing IgE antibodies that trigger a "complement cascade" of histamine. The histamine results in the red lesions of Hives that we see.

Symptoms
Red circular Hives/wheals -- red, raised welts. Sometimes the welts will "melt into each other," giving the appearance of areas with large raised red swelling
Itching is usually severe, but occasionally absent.
Swelling of tongue, lips, eyelids, palms, soles, and genitals in more severe attacks. Breathing problems may also occur.

Cause
Allergies:
Food, e.g., shellfish, nut, strawberries
Environmental, e.g., feathers, animal hairs, and cosmetics
Medications, e.g., penicillin, aspirin
Medications -- nonallergic form, e.g., Atropine, Pilocarpine, Morphine
Insect/bug bites and stings
Heat, Cold, Skin pressure
Infections such as Hepatitis B