They are usually made of titanium and they replace the roots of the tooth inside the bone. Other components for the implant are connector piece (abutment) and crown. The connector piece attaches to the implant and sticks out of the gum for the crown to be placed on it. Implants crowns are usually made of porcelain and they look like a tooth and function like a tooth.
If the implant is not an option and both sides of the missing tooth are stable, we can place crowns on the adjacent teeth that would be connected by a middle crown sitting on the gum. The dental lab fabricates the bridge and it would be cemented permanently on the prepared adjacent teeth.