They can occur on any part of the skin and usually disappear over 24 hours and then reappear in another part of the skin. Wheals are caused by histamine release from mast cells in the skin.
They can be accompanied by deeper swellings in the eyelids, lips and hands called angioedema.
It can be short lived but can become chronic and last for years.
Most cases of urticaria are not due to allergy.
Urticaria can be treated with anti-histamines, tablet treatments which alter the immune system and injection treatments which are very effective.