A crown involves only a single tooth. The tooth is prepared (shortened) to make space for the crown, which fits over the prepared tooth. The crown protects the tooth from fracture and can cosmetically improve it as well. A bridge involves multiple teeth and replaces a missing tooth. There are anchor teeth are either side of the missing tooth space which are prepared, just like they would be for a crown. Then the bridge is permanently cemented onto the teeth and connects them by filling the empty space.