Heroin is a narcotic which can be injected, smoked or snorted. It comes from the opium poppy grown in Southeast Asia (Thailand , Laos and Myanmar (Burma); Southwest Asia (Afghanistan and Pakistan) and Mexico and Colombia . It comes in several forms, the main ones being "black tar" from Mexico (found primarily in the western United States) and white heroin from Columbia (primarily sold on the East Coast).

Heroin can be injected, snorted or smoked. All of the methods of use can lead to addiction, and the use of intravenous needles can results in the transmission of HIV and hepatitis C.

Effects 

Euphoria, drowsiness, respiratory depression, constricted pupils, nausea.

Slow and shallow breathing, clammy skin, convulsions, coma, possible death.

Street names

Diamorphine, Horse, Smack, Black Tar, Chiva, Negra (black tar)

(Excerpts from justthinktwice.com)