Crack cocaine is the freebase form of cocaine that can be smoked. It may also be termed rock, hard, iron, cavvy, base, or just crack; it is the most addictive form of cocaine. Crack rocks offer a short but intense high to smokers. Crack appeared primarily in impoverished inner-city neighborhoods in New York, Los Angeles, and Miami in late 1984 and 1985. Common baking soda (Sodium bicarbonate, NaHCO3) is a base used in preparation of crack, so named after the sound made during its manufacture and when smoked.

Crack cocaine is popularly thought to be the most addictive form of cocaine, and one of the most addictive forms of any drug. However, this claim has been contested: Morgan and Zimmer wrote that available data indicated that “…smoking cocaine by itself does not increase markedly the likelihood of dependence…. The claim that cocaine is much more addictive when smoked must be re-examined.” They argued that cocaine users who are already prone to abuse are most likely to “move toward a more efficient mode of ingestion” (that is, smoking).

Reference: http://en.wikipedia.org/wiki/Crack_cocaine