Armpit odour, or axillary malodor, usually begins when you reach puberty, when your glands start working. There are two kinds of sweat glands in the human body, the apocrine glands, which secrete a milky fluid from the hair follicles, and the eccrine glands, which are the source of most perspiration. The sweat from the eccrine glands is water, which is used to cool the body by evaporation and to moisten the friction surfaces of the body.
Most axillary, or armpit, sweat is odourless, and comes from the eccrine glands. The small amount of apocrine secretion in the sweat, however, causes the problem.
Although fresh apocrine sweat is also odourless, it develops its characteristic smell when the sweat is broken down by the hair and skin cells in the armpit, and then eaten by bacteria.
It's actually the bacteria that make the smell, releasing a chemical called 3-methyl-2-hexenoic acid.
Antiperspirants act like a second skin under your armpit, preventing perspiration from being released. Without the sweat, the bacteria can't grow. Some people don't like the crusty feel that these leave on the skin.