Besides having dental implants, there are two main ways to replace missing teeth. The first is with a removable false tooth or teeth - a partial denture. The second is with a fixed bridge.
A dental bridge is usually used where there are fewer teeth to replace, or when the missing teeth are only on one side of the mouth.
Bridges are usually made of a precious metal base which is then covered with aesthetic porcelain to look like a real tooth or teeth.
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.