When you go to the dentist they make a map of your teeth, which you have, size, position etc. This is recorded in a standard format which can easily be indexed.
They can then check the same layout on the body of the "victim" and match that with existing records.
There are of course a number of problems, firstly they are not unique to the degree DNA or finger prints are, secondly it obviously requires that the person has been to the dentist recently.
It's generally the case it is used to identify a body who the suspect as being a particular person, i.e. they know it is Joe Blogs, so they check Joe Blogs' dental records to confirm, rather than there being a central database.
The important thing is that a skull/teeth will not burn or decay at the same rate as flesh, therefore if a body has been found which has extensive damage this is an easier way to identify.