Humans have two main types of sweat glands. Eccrine glands cover most of the body and produce a salty, watery sweat. You likely have between 2 and 5 million of them [source: Encyclopaedia Britannica]. Apocrine glands are found typically under our armpits and groin. They grow and begin to function after puberty. Apocrine glands secrete a milky type of sweat, the main ingredient of which is water, along with proteins, lipids, fatty acids, cholesterols and iron-containing salts [sources: Yamazaki et al.].
When all that sweat encounters the bacteria brimming on your skin, odors start to rise. The bacteria, such as Staphylococcus epidermis and S. aureus, break down the sweat and generate smelly byproducts. So, in a sense, we cheated a bit on this one. The bacteria don't pose the problem, the byproducts do. We'll get to a few of them, too.