A friend of mine is having a dental implant placed. What is this?

Dental implants are a comparatively recent development. An implant is basically a titanium screw, upon a false tooth is supported, which 'screws' directly into the jaw bone. Implants are used as anchors for fixed false teeth (like a crown or bridge as described above) or to provide support for a removable denture.  

Implants are very successful, but they are not suitable for everybody. You must have enough bone to place the implant into for example. Implants are quite expensive, but where they can be used, they are often the ideal solution to replacing missing teeth.