Bridges are used to replace missing teeth using the teeth next to the gap as support. It is basically a line of joined crowns. Usually the ones on either end are glued onto the supporting teeth and they have a "floating" tooth joined to them that fills the gap. A bridge is fixed in the mouth and is not removable.
Bridges can be made to replace one or several teeth and are usually made from metal with porcelain attached to the surface to make it look natural. Just like a crown, the supporting teeth have to be filed down to make enough space for the laboratory technicians to make a "cap" to cover the surface. However, because several teeth will be linked together this can be quite a difficult procedure.
Once the dentist is happy with the preparation, they will take a mould of the teeth and send this to the laboratory technicians who will use this to make the bridge. The technicians work closely with the dentist and sometimes may meet with you to make sure they produce replacement teeth that look natural and are exactly what the patient wants.