All saline and silicone gel breast implants have an outer shell that is made from solid silicone. As the name implies, smooth implants have a smooth, slick surface. Textured implants have a rough, clingy surface.
Right after breast augmentation surgery, most implants look too high and too tight. I joke with patients that their implants will look like decorative shelving for the first couple weeks. Over the course of the first month or two, the implants typically drop and look more natural, though final results do not occur for about a year.
Smooth breast implants are "slippery," and their weight will cause them to fall after surgery. This is desirable when the implants are below muscle, and the pectoralis major muscle has adequate bulk to hold up the implants. As a result, smooth implants are used in the majority of routine submuscular breast augmentations.
Textured breast implants tend to cling to the surrounding tissues, making them more resistant to falling after surgery. This can be good for women whose breasts and/or chest muscles provide inadequate implant support. Textured implants placed in a subglandular (above muscle) position are also associated with a mildly lower risk of capsular contracture (excessive firmness of the implants) than smooth implants placed above muscle.
So which type is better? I routinely use smooth implants for most breast augmentations. But I always have textured implants available, since you never know when you might need them.