Fentanyl is a very potent opioid that is used (along with a range of fentanyl analogues) to adulterate - or simply mimic - more desirable (and safer) narcotics.
They are sold as heroin or pressed into tablets that look like pharmaceutical opioids, and seem to be frighteningly common in parts of north america and parts of Europe.
Fentanyl is really dangerous stuff because it is so potent that measuring a safe dose is beyond the capabilities of most clandestine drug manufacturers and distributors. Powders and pills containing fentanyl (but passed off as heroin or pain pills) can be massively over-dosed, which has lead to the deaths - and resulting publicity - surrounding fentanyl and its analogues being misrepresented as opiates and opioids.
Most people find fent to be far less desirable than other opiates; not just for its deadly potency, but also its short duration and reputation for pushing people's tolerance to ridiculous levels.