A bridge is usually used where there are fewer teeth to replace, or when the missing teeth are only on one side of the mouth. Dental bridges are usually made of a precious metal base. If the bridge will show, porcelain is then bonded to the base. Sometimes, there are other non-precious metals used in the base to reduce the cost.
You should replace missing teeth for a number of reasons. Your appearance is one reason. Another is that the gap left by a missing tooth can mean greater strain is put on the teeth at either side. A gap can also mean your 'bite' is affected, because the teeth next to the space can lean into the gap and alter the way the upper and lower teeth bite together. This can then lead to food getting packed into the gap, which causes both decay and gum disease.