Today we use two major types of breast implants: saline implants and silicone implants.
Current [silicone] implants are what we refer to as cohesive gel implants, and they are more like jello; They have a very soft and natural feel and behave more like breast tissue than saline breast implants. This is the chief advantage to silicone breast implants. They have a softer more natural look, and feel and tend to be lighter than saline breast implants.
Saline breast implants are excellent for patients with thick young skin, and no problems with sagging. Patients who have had children and complain that their breasts are deflated with sagging tend to have tissue and skin that's thinner. Because of this, a saline implant may be more visible and therefore not give as natural a result as a silicone breast implant which is much more tissue like.