The most common type of cases we see is a single missing tooth or a few missing teeth.  The example below shows a missing front tooth originally from a sporting injury.

Implants can also be used to stabilize dentures.  This is very common in lower jaw where it is almost impossible to have full dentures that do not move around while talking and eating.
Sometimes all teeth are missing on the upper jaw or lower jaw or even both jaws. 
The example below shows missing upper teeth due to decay over many years.

Not all teeth are lost in our adult lives.  In some cases, teeth can be congenitally missing which means they never developed at all as a child.  The example below shows two congenitally missing lateral incisors.  Spaces were made by the orthodontist so the teeth will be the correct size.  Sometimes, the orthodontist completely closes the spaces but the canine teeth are often too large or are darker.