There are different types of dentures, but they share their common function. They replace teeth that have become loose or been lost due to bone loss. When bone loss around the roots of teeth is great enough to loosen them or let them fall out, it's time for dentures. Relax. No one enjoys losing their natural teeth, but you can still eat and talk regularly.
The entire mouth is examined and a determination is made as to which teeth will have to be removed, and which will remain. The loose teeth are then extracted. Dentures are fitted to go over or around whatever teeth remain in the mouth, depending on the type.
There is an adjustment period after dentures are placed in the mouth, and it can take some getting used to. But once accustomed to the dentures, all the normal functionality and appearance return and one just carries on as usual. Often implants can used to further stabilize the dentures.