What is the difference between a matopexy and mastectomy?

Mastectomy means removal of all milk producing tissue with surgery. Mastopexy means pulling breast tissue together in a lifted fashion, but not trying to remove any particular tissue. If mastopexy is combined with reduction (removal of some tissue to make the size smaller) or capsulectomy (removal of scar tissue that has formed around an implant), then the final change in the breast can be smaller and lifted. For some women with not very much tissue to begin with, taking out the scar tissue around an implant can leave very little else behind. If so much will be gone, some women decide to have all the milk producing tissue taken because of risk or to simplify things. There are increased risks of poor wound healing and a more artificial appearance if mastectomy Isn't performed well. 

Mastectomy is removal of all the breast tissue - generally while preserving the skin and the nipple. This is indicated in some patients with cancer or a high risk of getting cancer. Mastopexy is rearranging the tissue of the breast - such as a lift or to fill in a defect (possibly from lumpectomy) but no breast tissue is actually removed. Droopy or asymmetric breasts may benefit from mastopexy. Hope this helps clarify things.