Generally, there are two different types:

Semi-rigid positionable rods have an inner core made of stainless steel or interlocking plastic joints. A man can bend the penis up when he wants an erection and bend it back down when he’s finished with sexual activity.

Inflatable implants use a pump system. When a man wants an erection, he activates a pump placed in the scrotum. This action sends fluid (usually a saline solution) into inflatable cylinders placed in the penis.

Inflatable implants can have 2 or 3 parts. With 2-part implants, the fluid is held in the back part of the inflatable cylinders. With 3-part versions, the fluid is contained in a separate reservoir.

Nowadays, 3-part inflatable implants are the most common.

Both semi-rigid rods and inflatable implants are completely concealed within the body. Nobody can see them.