Balloons are used to hold heroin (usually low grade H). A dealer will use un-filled balloons (the multi-colored, water balloons, most of the time) to place a small, universal weight, amount of heroin into. The weight of the heroin inside each balloon can vary depending on who packaged or "tied" them. The amount is usually about .1grams per balloon. People buy them in packs of 10 or more, and rarely buy/sell a single "balloon." Rumor has it, the reason heroin was originally put into these balloons, was so you could swallow them if need be, and not need medical attention.
J.Do: "Yo mang, can you hook me up wit sum dope, maybe sum of that black tar you had last week?"
J.Do2: "Nah fool, but I do gots some bomb BALLOONS ima sell fo cheap!"