Here,  a tooth has broken off at the gum line.  The tooth has had a root canal and a post is placed in the top part of the root canal space.  This provides something to hold the crown in place.  The part of the post-care that is in the tooth is called the post and the part in the crown in the core.

Replacing a Missing Tooth:
This drawing shows a missing lower molar.  The missing tooth is replaced by a bridge which is really  a crown on each tooth with a "dummy" crown in the middle.   The crowned teeth of the bridge are called the abutments and the part of the bridge that replaces the missing tooth is called the pontic.  Bridges are made in the same basic way crowns are made. The charge here would be for three units.  In other words, if a crown, or single unit, costs $1000, a bridge might cost $3000 since it involved three units rather than one.  This is just an example. Some offices charge more per unit when doing bridges.

Here, the patient probably could have had an implant rather than a bridge, but an implant would require surgery and an implanted artificial root in the toothless area with a crown covering it.
An alternative here might be an implant.  Placing an implant may save having to cap the teeth on either side of the space.   On the other hand,  placing a bridge will save having an implant surgically placed in the bone!  See Tooth Implant. Your dentist should give you all possible alternatives, explain the risks and let you choose.