Heroin is an illegal, highly addictive opioid that is typically injected, but can also be snorted or smoked. It is typically sold as a white or brownish powder, but also sold as "black tar" heroin which is sticky like roofing tar or hard like coal.
Fentanyl is an opioid drug that is 50x more powerful than heroin. Medically, it is used to treat severe pain and for surgeries. It is now being made illegally, sold on the streets for its "heroin like" effect and often mixed with heroin, cocaine, marijuana, methamphetamine, imitation or fake pills and other drugs - with or without the individual who is taking its knowledge.
Due to its high potency, even the smallest amount can lead to an overdose. Illegally made, or distributed Fentanyl is now the most common drug involved in drug overdose deaths in the United States.