Joy dust was a drug that instilled false feelings of happiness. It made addicts completely uncaring about what happened to them or to others, and eventually killed them.
It was prevalent on Turkana IV, sold by the drug-lords that dominated the world. They used joy dust to keep their sex slaves docile.
Tasha Yar was born a joy dust addict in 2337 due to her mother's addiction. Her mother continued to feed her own baby the drug to keep her quiet, until she couldn't afford enough for both of them, so she saved the drug for herself. Tasha's earliest memories were of withdrawal pains. In 2342, her mother abandoned her at only age 5. An old woman named Yar who cared for Tasha afterwards kept her away from the drug. (TNG novel: Survivors)