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 Colombia (primarily sold on the East Coast).

In the past, heroin was mainly injected; because of the high purity of the Colomban heroin, many users snort or smoke heroin. All of the methods of use can lead to addiction, and the use of intravenous needles can results in the transmission of HIV.

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)