Fentanyl patches are a transdermal narcotic used for pain management. Folks get them prescribed after surgery or for chronic pain, etc.
The effect is like any opiate - vicodin, percacet, oxycontin, etc. It's a synthetic form that's made in a lab. Think of a nicotine patch, but with heroin basically. The idea (when used properly) is that you get a consistent, low dose of narcotic over a longer period of time.
They've been around for several years, not sure how long exactly. Since it's a narcotic, I'm sure folks have been abusing them since their creation.