All about perspiration > What makes sweat smell?

Your body has two different kinds of sweat glands: eccrine and apocrine. By the age of three, you've developed 2-5 million eccrine glands all over your body, but only 25,000 of these glands (less than 0.5%) are found under the arms. These glands secrete a watery sweat directly onto your skin, especially in response to heat and emotion. The sweat secreted by our eccrine glands is naturally odorless - it's water containing a little salt.

The apocrine glands, located in the underarm and genital areas, usually don't become active until puberty. In response to stress, this sweat clings to hair follicles, and is broken down by bacteria. This is the kind of sweat you use deodorant for.

For your information, the apocrine glands or odor glands are those which animals use for sexual identification.