The base or footprint of the implant can be either round or elliptical. The profile of the implant can have the point of maximum projection in the center of the implant or toward the bottom of the implant (giving more volume at the bottom of the implant). Then all of these come in different projections. A board certified plastic surgeon (member of the American Society of Plastic Surgeons) can discuss this with you in detail and how the implant shapes would benefit you, at your consultation.
There are round implants and teardrop shaped implants. Then the shapes can change with profiles- basically low, medium, and high.