Here are Allergan Natrelle™ smooth (top) and textured surface saline breast implants (filled with air for illustrative purposes). 
Here is Allergan Natrelle™ smooth surface silicone gel breast implants.
Breast Implant Shells
Breast implant shells may have a smooth or textured surface. As a natural reaction to any device placed in the body, scar tissue will form around the breast implant surface, creating a soft tissue capsule.
In some women, the capsule can tighten and squeeze the implant. This occurrence is called "capsular contracture".
Textured Breast Implants
Textured breast implants were designed to reduce the chance of capsular contracture. Some information in the literature on small numbers of patients suggests that surface texturing reduces the chance of severe capsular contracture.
Clinical information from studies of a large number of women with breast implants shows no difference in the likelihood of developing capsular contracture with textured breast implants compared to smooth-surfaced implants.
Smooth Breast Implants Are Preferred
Dr. Connall usually recommends smooth breast implants when round implants are used. Smooth implants are softer and feel more natural than textured breast implants. One is less likely to feel the edges of smooth implants and smooth implants generally cause less visible implant rippling compared to textured implants. In addition, with some models of breast implants, the textured versions have a higher chance of failure (leakage) compared to the smooth versions. Saline and silicone filled breast implants are both available in smooth and textured shells.
Teardrop Implants Must Be Textured
All teardrop implants require a textured surface to prevent rotation of the implant within the breast. The textured surface creates a "velcro effect" as the surrounding tissue adheres to the implant shell. This tissue adherence holds the implant in a fixed position. Keeping a teardrop implant in the correct orientation is necessary to maintain a normal breast appearance. If a teardrop implants rotates (for example, up-side-down) then the breast will be misshaped.