The teeth that you see in your mouth are connected to you by the root of the tooth.
An implant is simply replacing the natural root with an artificial one and then attaching a tooth to the new artificial one.
Implants can replace single teeth, multiple teeth, or be the way of attaching a loose denture in the mouth with a more firm grip.
Almost anyone is a candidate for implants. Certain medical conditions and drugs that people are taking may present problems but other then that implants are getting to be a pretty universal from for replacing missing teeth.
Most insurances cover implants. Occasionally there are some insurances that are still in the dark ages and don't cover implants so you need to check with your particular company first.
There is no dental specialty that is in charge of placing implants. They are done by general dentists, periodontists, and oral surgeons. Just make sure that the person placing your implants can show you some before and after pictures of work they have done and then can adequately explain the procedure you will be going through so that you will fell comfortable with what will be happening in your mouth.