Jez is right - implants can be used for single teeth, for example in a younger person who'd had a tooth knocked out, or to support a bridge or a denture. They tend to be made of titanium, which fuses to bone. Some use magnets, others act more like a press-stud. I ain't an implantologist, for some reason they really leave me cold, but my boss is and between us we'll answer any questions!
Rach x