If you've ever worked up a sweat and found yourself wondering about that particular smell emanating from your body, you've experienced some level of bromhidrosis. It's the medical term for body odor, or "B.O." Sweat itself doesn't actually smell like anything. Body odor comes from bacteria that live on the skin in areas where a specific type of sweat gland is located. Apocrine glands are in the areolae, genital area and armpits, and (as opposed to the eccrine glands everywhere else) the sweat that they produce is full of protein and fatty acids. Apocrine sweat serves as food for the bacteria, which in turn create that distinctive odor when they metabolize the sweat.
We all have apocrine sweat, so why do some people produce very little body odor while others get really stinky really quickly? Genetics plays a part; some people just have sweat that's more attractive to bacteria, or have more sweat-eating bacteria. Diet comes into play -- eating lots of garlic, for example, can result in greater body odor -- and some medications can cause an unpleasant smell as well. Men also tend to have stronger body odor than women.
If you're worried that your body odor is abnormally strong, try using an antibacterial soap and showering more frequently. A stronger antiperspirant/deodorant will not only help cover up the smell, but cut down on the sweat in the first place.