Most people who use cocaine do so for recreational use, to get the feeling of being "high". The sale and possession of cocaine are strictly controlled in most jurisdictions, however, there are still some "drug pushers" who are selling cocaine discreetly and that's where teenagers get their supply of cocaine.
Crack is cocaine that has not yet been neutralized by an acid to make the hydrochlorized salt that makes up cocaine. Crack is a form of cocaine, and it takes form in a rock-like crystal that can be heated and smoked. Crack got its name from the "crackling" sound it makes when it is heated.
Cocaine and crack users are candidates for experiencing acute cardiovascular or cerebrovascular emergencies such as stroke or heart attack. Excessive use of cocaine and crack leads to sudden death.
Methamphetamines are stimulants that are referred to in slang as "meth" or "ice". Methamphetamine in powdered form is the hydrochloride salt form that is highly hydrophilic (absorbs water from the air quickly). The HCl salt is smokable and teens smoke this HCl salt during "pot sessions". "Ice" refers to methamphetamine that is grown in crystals. Although the HCl salt form of methamphetamine is already smokable, most people prefer the "ice" form because methamphetamine is easier to smoke if it is grown in crystals.
Anything used in excess is not healthy. Long-term use of stimulants, whether they are legally prescribed or not, may lead to the degrading of one's health. Stimulants can become very addictive to the point that some individuals use them compulsively.