Dental Bridges are restorations cemented in place that are used to replace missing teeth. Getting a bridge is just one of several options for replacing missing teeth (other options are implants and dentures). The option for the material of bridges is the same for that of crowns. However, the longer the span of the bridge (space between abutment teeth), the more flex it has and the higher the risk for porcelain fracture.
Bridges work best when there are solid teeth on either side of the missing tooth. The abutment teeth act as anchors for the bridge, and may be natural or implants. The pontics are the portions that replace a tooth. Depending on the quality of the abutment teeth, and the number of pontics, the preparations on abutments may be minimally invasive (like a veneer) or more invasive (like a crown). The key for longevity of bridge work are cleanliness, and strong abutment teeth.
When making the decision to get a bridge, it is very common for people to consider its lower cost and better coverage by insurance, than that an implant. We tend to caution against this line of thinking, as it is not uncommon for another tooth to be lost when a bridge finally does fail, and the future replacements cost is often higher than that of an implant done from the start.
Forces also need to be taken into account when deciding on a bridge. You have to remember that for each tooth being replaced, all that biting-force is now added to the anchor teeth. So if two teeth are being replaced using two anchor teeth, those teeth are now going to experience double their normal force, which can severely compromise their life span.