"Heroin is a synthetic opiate drug that is highly addictive. It is made from morphine, a naturally occurring substance extracted from the seed pod of the Asian opium poppy plant. Heroin usually appears as a white or brown powder or as a black sticky substance, known as 'black tar heroin.'"Source: National Institute on Drug Abuse, InfoFacts: Heroin Rockville, MD: US Department of Health and Human Services http://www.nida.nih.gov/infofacts/heroin.html
The LD50 for heroin is generally believed to be around a third of a gram. That means that if you were to (hypothetically!) give random people a third of a gram of heroin each, about 50% of them would die and the other 50% would survive.
People have overdosed on amount as small as 75mg, which is typically what you might find in a single $10 "dime bag" of street heroin. Street heroin ranges in potency pretty wildly, but on the East Coast, most powder heroin falls between 50% and 90% pure. On the West Coast, Black Tar is usually between 10% and 30% pure, but much cheaper than the powder by volume.