There are many ways to replace one or more missing teeth:
The traditional way to replace missing teeth is with a removable prosthesis (a partial) that hooks onto the remaining teeth for support. As you can imagine, this appliance would have a limited stability. Another method to replace missing teeth is by a bridge which is cemented on the neighbouring teeth. This method is usually better tolerated by the patient but it can only be used to replace 2 or 3 missing teeth, or else the occlusal load becomes excessive.
The new revolutionary method to replace missing teeth is by implanting a titanium screw that will then support the future prosthesis. In the 1950s, Dr. Branemark of Sweden proved that titanium is biocompatible with living bone. With this discovery came the era of implantology.
An implant is a screw which is used as a metal support in the bone to simulate the root of the tooth. To replace the supra-gingival part of the tooth, a crown is cemented onto this implant. Of course, a larger and longer implant would be able to support a heavier occlusal load and the implant would then have a better prognosis. However, the width and height of an implant depends on the amount of bone structure available in the site.