Dental crowns are fixed prosthetic devices that literally bridges the gap created by one or more missing teeth. Just as goes with all kinds of bridges, the dental bridge needs support or anchoring as well. Thus two natural teeth at the two ends gap created by the missing tooth/teeth. In cases where a natural tooth cannot quality to be an anchoring tooth, an implant may be used.
More options are available that you can learn from the dentist on you visit. We at i2m Dental mostly recommend ceramic or PFM based bridge.