A solid, flexible silicone shell filled with silicone gel. The correct name is silicone. That thing that some folks call silicon is a completely different material, often found in computer chips.

Pretty simple really, until you stop to consider that there's the original silicone gel (the type which originally got banned by the FDA) and the newer cohesive gel (a.k.a. "gummy bear") implants. It's commonly believed that the original formulation still feels the most natural out of all implant designs. However, the newer cohesive gels have been found to be remarkably realistic; they're nowhere near as firm as Jell-O, and yet if you were to cut a wedge out of one of these implants, the gel would stay in place. That's right, the gel won't ooze out like Jell-O on a hot summer day. Hence the name "cohesive", because it has a strong tendency to remain whole.