A breast implant is a silicone shell filled with either saline (salt water) or a silicone gel. If you haven't had a skin-sparing mastectomy, you'll probably need a tissue expander to stretch or expand the skin that remains after the mastectomy to make room for an implant.
The expander is basically a balloon-like bag inserted beneath your skin and chest muscle. It has a tiny opening through which liquid (saline solution) can be injected. During a period of weeks or months, your doctor will gradually fill it with liquid; as it expands, it stretches the skin above it into the shape of a breast. When it reaches the desired size and shape, it's removed in a surgical procedure and replaced by the permanent implant.
But in some cases, the expander itself becomes the permanent implant, and no further surgery is needed. In other cases, an expander is unnecessary and the permanent implant is inserted in the first surgery. In all cases, the nipple and the areola (the dark skin around the nipple) are reconstructed in a later procedure.
In a traditional mastectomy, the breast skin is removed along with the breast. As its name suggests, a skin-sparing mastectomy preserves all the breast skin, except the nipple and the areola. This eliminates scars on the breast and makes reconstruction easier. If you've had a skin-sparing mastectomy, you can choose one of the "flap" procedures or implants for reconstruction.