Dental crowns ("tooth caps") are one of dentistry's most versatile restorations. Beyond just offering a way to rebuild and strengthen damaged or compromised teeth, they also provide a way by which a tooth's overall appearance (color, shape and even apparent alignment) can be vastly improved.
A crown is a type of dental restoration that, when cemented into place, fully cups over that portion of a tooth that lies at and above the gum line. Since it encases the entire visible aspect of a tooth, a dental crown, in effect, becomes the tooth's new outer surface.
In comparison, dental fillings are restorations that fill in or patch just a portion of a tooth. Other terms that are used to refer to crowns are "dental caps" and "tooth caps."
Crowns can be made out of porcelain (meaning some sort of dental ceramic), metal (a gold or other metal alloy), or a combination of both (porcelain-fused-to-metal).
A dentist might recommend placing a crown on a tooth for a variety of reasons but, in general, most of them will usually fall within one of the following basic categories:
Restoring / improving the shape of a tooth. It's not too hard to envision how a dental crown might be used to restore, or even improve, a tooth's shape.
When a crown is cemented into place, it becomes, in essence, the tooth's new outer shell. And although they must stay within certain guidelines, it's easy to seen how placing a crown gives a dentist great control in defining the overall shape of a tooth.
It is conceivable that a dental filling, as an alternative, might be placed instead. Crowns, however, offer a big advantage due to the fact that they are fabricated "away from your mouth."
By this we simply mean that dental crowns are fabricated in a dental laboratory (by a dental technician using plaster molds your teeth). Dental fillings, in comparison, are created "in your mouth" by way of your dentist placing the filling material directly upon your tooth.
A dental laboratory technician has opportunity to visualize and examine all aspects of your bite and jaw movements from a variety of angles, and then sculpt your dental crown so it has the ideal shape. In comparison, when a dentist places a dental filling they have far less control over the final outcome because it is often difficult for them to visualize, evaluate, or access to the tooth they're working on.
Dental crowns are routinely made for teeth that have broken, have large portions missing due to tooth decay or have had root canal treatment. This is because beyond just restoring the tooth's shape, a crown can provide a reinforcing and strengthening effect for it too. (This is especially true for all-metal and porcelain-fused-to-metal crowns.)
Dental fillings (amalgam or bonding), can't typically provide a substantial reinforcing effect for a tooth in the same way.
Because dental crowns encase the visible portion of a tooth, any porcelain crown (all-ceramic or porcelain-fused-to-metal ones) can be used to enhance or idealize the cosmetic appearance of a tooth.
Taken to an extreme, this technique can be used to radically improve the appearance of a person's smile. In fact, in decades past it was common to hear that some movie stars had had their teeth "capped." This simply meant that they got their perfect "Hollywood" smile by way of having dental crowns placed.
Actually, getting your teeth "capped" just to improve their appearance can, in most cases, be a very poor choice. Dental crowns are best utilized as a way to make a cosmetic change when the crown simultaneously serves other purposes, such as restoring the tooth to its original shape or strengthening it.