A dental implant is a replacement for a missing natural tooth. As a tooth crown requires some anchorage or support on which it is placed, this support can be taken in 2 ways :

Either from the adjacent natural teeth (crowns & bridges)
From a dental implant
An implant is a titanium-based material which is in the shape of a tooth root. It is inserted into the bone below the gums, where it is then left to integrate with or fuse with the existing natural bone. This is called osseointegration. A crown or the superficial part of a tooth, which is what is seen in the mouth, is then placed onto this implant. Implant treatments may require one or more additional supportive treatments as well depending on the clinical situation.
Implants come in a wide variety of choices in terms of companies & sizes. Types of implant treatments vary based on the requirements of the patient.

WM implants
WM implant supported bridge

An Implant supported Bridge/Fixed Partial Denture

There may be:
Single tooth implants
Multiple implants supporting a fixed bridge
Multiple implants each for a single tooth
Full mouth Implants: Fixed crowns OR Removable dentures