RFIDS are subdermal implants, they're about twice the size of a grain of white rice. They're inserted using a very large needle after local anesthetic numbs the area; usually your upper arm. They can be used for a lot of things, not just tracking; identification, storing information, granting access to secure areas, etc. There are health risks associated with them, but Mythbusters has taught us that it's very unlikely that they'll be pulled out or explode if you're in an MRI. This is all I know without the google.