Crowns are a restoration used to strenghthen or improve the shape of a tooth. Crowns will be generally be a solution for teeth that are broken, worn, or partially destroyed by tooth decay.
Crowns are "cemented" onto an existing tooth and can either cover a part of the tooth, or cover the tooth completely. Esentially, the crown becomes your tooth's new outer surface. Porcelain crowns are the most popular because they mimic the translucensy of your natural teeth and are very strong. However, crowns can also be made of metal and gold.
Onlays, or partial crowns are used when there is not enough tooth strength remaining to hold a filling. Unlike fillings that are placed directly into the mouth, a crown is created in a lab from your tooth impression which allows a technician to examine all aspects of your tooth and bite. Your crown is then created for you so that your mouth moves comfortably once the crownn is place.