What is popularly known as 'cocaine' is actually the chemical compound 'Cociane Hydrochloride'. This substance is nuetral, the 'Cocaine' part being a base and the 'Hydrochloride (Hydrochloric Acid)' being an acid. It is impossible to smoke Cocaine Hydrochloride because upon being heated, it simply decomposes.
So, some brilliant dopehead realized that the way to make it possible to smoke Cocaine is to 'free the base'. To do this, you mix 1 part cocaine and 1 part baking soda (carbonate of soda) with water. The hydrochloric acid detaches from the cocaine in order to react with the baking soda, producing water vapor and salt. Wait for the water to evaporate, and you have a 'rock' or crack, usually white, yellow, or pea-colored. You will wind up with slightly more crack than you had cocaine.
Street-crack is usually adultered and contains many chemicals in it besides freebased cocaine. To at least partially remove these chemicals, set your rock on a hard surface, heat with a lighter until melted, and then scrape off your slightly more pure crack with a razor blade when it dries.
Crack is much more addictive than cocaine, because it absorbed faster. The lungs have a much larger surface area than the mucas membrane of your nose, so immediately after taking a hit of crack, you'll get the high. It's gone almost as fast, leaving all the dopamine in your nerve synopses screaming "MORE CRACK!" You will then smoke more crack. Unless you are a millionaire, you will probably run out of crack before you die of chronically high blood-plasma levels, so you will most likely start off by selling all your clothes and your house, before resorting to crime and prostitution in order to get a fix.
Have fun, kids.
In a famous study, a chimpanzee hit a lever to get a hit of crack. After the second hit, he had to hit the lever twice for a single hit. After that, four times, and after that, eight times. The chimpanzee eventually hit the lever over 30,000 times for a single hit of crack. Then he died.