What is a Mastectomy?

Mastectomy is surgery to remove all breast tissue from a breast as a way to treat or prevent breast cancer.
 
Mastectomy is used to remove all breast tissue if you have breast cancer or are at very high risk of developing it. You may have a mastectomy to remove one breast (unilateral mastectomy) or both breasts (bilateral mastectomy).

What to Expect:

Mastectomy is usually performed under general anesthesia, so you’re not aware during the surgery. Your surgeon starts by making an elliptical incision around your breast. The breast tissue is removed and, depending on your procedure, other parts of the breast also may be removed.
Regardless of the type of mastectomy you have, some breast tissue and lymph nodes will be sent to a laboratory for analysis.
If you’re having breast reconstruction at the same time as mastectomy, the plastic surgeon will coordinate with the breast surgeon to be available at the time of surgery.
One option for breast reconstruction involves placing temporary tissue expanders behind the chest wall muscle. These temporary expanders will form the new breast mound.
For women who will have radiation therapy after surgery, one option is to place temporary tissue expanders in the chest to hold the breast skin in place. This allows you to delay final breast reconstruction until after radiation therapy.
If you’re planning to have radiation therapy after surgery, meet with a radiation oncologist before surgery to discuss benefits and risks, as well as how radiation will impact your breast reconstruction options.
As the surgery is completed, the incision is closed with sutures (stitches), which either dissolve or are removed later. You might also have one or two small plastic tubes placed where your breast was removed. The tubes will drain any fluids that accumulate after surgery. The tubes are sewn into place, and the ends are attached to a small drainage bag.
 
Source: http://www.mayoclinic.org/tests-procedures/mastectomy/basics/definition/PRC-20012749?p=1