These are removable replacements for missing teeth . They are typically made of acrylic but can also incorporate metal in their  design. There are two main types:

Complete Dentures

These are made when a person is missing all of their teeth.

Removable Partial Denture

These are made when a person is only missing a portion of their original dentition.
These attach to your teeth by either metal clamps or special acrylic clasps.