Hives are caused by an allergic reaction to the skin that often result in raised, red bumps that burn and itch. They can appear anywhere on the body in different sizes, and last anywhere from a few hours to a day, but can also com back for months if continual exposure to the allergen occurs. Treatment ranges from cool compresses to oatmeal baths, to antihistamines to oral corticosteroids. Once they're gone, though, sometimes scarring can occur, usually from over-zealous scratching.
Ask your dermatologist about OTC or prescription ointments that will help fade scars, and if any mark will be left behind. Be sure to weigh your options such as cost, length of treatment and possible side effects.
Ask if injections could be helpful. For scars that are raised, steroid injections might be useful, while collagen can fill in pitted skin (collagen is not permanent, though, and will require a long-term commitment.)
Consider surgery for deeper scars; sometimes people will thin skin and recurrent hives scar their skin deeply from prolonged itching. Possible treatments include dermabrasion, which is relatively light and painless, to laser surgery. Choose an experienced cosmetic surgeon who has done the chosen procedure many times, and be sure to ask to see before and after photos.
Wait it out. Scars usually fade over time, especially if not severe. If there is some discoloration, you can buy an OTC pigment-lightening gel or ask your doctor for one.