What a crown is
A crown covers the whole tooth down to the gum line and takes over the job the outer tooth structure used to do. It is made from porcelain, ceramic or a metal alloy, and shaped and shaded to sit alongside the teeth either side of it.
When a crown is used
On a tooth that is cracked, or so decayed that a filling would have nothing solid to hold on to. On a tooth that has just had a root canal, because a treated tooth is more brittle than it was. And as the anchor at either end of a bridge, or the visible part of an implant.
What happens at your appointment
Two visits. At the first the tooth is shaped so the crown will fit over it and an impression is taken, with a temporary crown to protect the tooth in the meantime. At the second the finished crown is checked for fit and bite, then cemented.
Looking after it
A crown itself cannot decay, but the tooth underneath it still can, at the margin where the two meet. Keeping that edge clean is most of what makes a crown last.
