Implants are any artificial objects used to enhance or assist a human in some fashion.
Fine. According to the Wikipedia entry about them, the most common types of implants are pins, rods, screws, and plates. You know, the reason your relative/friend/guy in front of you is having to take off his shoes and get pulled to the back room for a strip search at the airport.
Other types of implants include...
Implants for old people. These involve pieces of metal, primarily stainless steel or titanium, placed onto and/or through bones to help them heal, or, in the case of old people, help them retain their actual skeletal structure. This category, of course, ecompasses the pins and such previously talked about.
Of course, there are also implants that replace parts of or entire bones, the most common being a hip prosthesis. This one is usually used for, you guessed it, old people (or young people whose weak bones wouldn't have allowed them to live in earlier, simpler times).
Artificial teeth. Dentures, for the purposes of this article, aren't implants, they're external prosthetics. Those have been around for quite some time. Actual embedding an object into the gums to act as a new tooth was thought to be a fairly recent thing to do. However, recent archeological evidence shows that the Mayans may have been using tooth implants as far back as 600 A.D, when a mandible from a burial site was found with three implants, taking the form of tooth shaped pieces of shell. How the Mayans performed this seemingly horrifying procedure is not known.
Electrical Implants and Bio-Implants are by far (forgetting about silicone for a bit) the coolest implants (though the titanium plates would be cool if you could opt for a full skeletal implant). These include pace-makers (for old people, cool in that you could kill someone by removing a battery), cochlear implants (electrical hearing aids embedded in your skull, kinda coo, for old peoplel) and brain implants (used in current studies for developing cooler implants and prosthetics). Bio-implants are like transplants in that they use organic tissue, but they are manmade, so they're implants. Those include things like "artificial" heart valves and corneas (these may also be used primarily for old people....)
Clearly, the use of implants on our elderly will at some point spawn a generation of enhanced geriatrics who will throw off their shackles and rise against the oppression of the "whippersnappers". This will culminate in an actual battle between the robotically enhanced, superpowered seniors, and the inferior, perhaps somewhat enhanced youth with their still useful, and thus unaltered, meatsacks. The Old will will destroy the young, and go back to the old cultural norms they were used to. It will be later that they realize they cannot further the human race, since they're old, and can't do make babies anymore. With all the young, fertile people gone, the Old will be unable to maintain their robot forms, and will not have the capacity to create new ones. Slowly, they will wither and die. The rusting metal and decaying flesh will be all that remains of humanity.......