Compared to fillings which just cover a small portion of a tooth, a crown (or cap) encases the entire visible portion of a tooth. In effect, the crown acts as the tooth's new outer surface. A dental crown is used when a tooth is broken or decayed to such an extent that fillings aren't able to repair the problem. The crown is able to provide a protective shell around the damaged or decayed tooth to strengthen it, as well as to improve the appearance of the tooth. They can also help restore a tooth to it's original shape, are used commonly for teeth that have been broken. While crowns come in different materials, the most common crowns typically have some mixture of porcelain/zirconia/lithium disilicate in them to give them a look and feel similar to a natural tooth.
How it's done
The first visit to your dentist involves reshaping the tooth and taking impressions to create the crown. Typically a portion of your tooth will have to be removed for the crown to fit properly. After the dentist reshapes your tooth, he will use a special material to create an impression of it. This impression will be sent to a dental laboratory to be made into a permanent crown. Before sending you home, the dentist will provide you with a temporary crown to cover your tooth in between visits.
When you return, your dentist will have received the permanent crown from the laboratory. They will remove the temporary crown and fit the new permanent one. Before cementing the permanent crown in place, they will ensure that it fits comfortably and matches the color of your teeth.