Sweat 101

Stress, hormones, emotions, and physical activity or exercise activate sweat glands. When this happens, the brain sends a message to the body telling it to cool you off with your body’s natural coolant—sweat. It’s your body’s way of maintaining your core temperature.
Find out more about why we sweat from WebMD.

In some people, the sweat glands overreact to stimuli and produce more sweat than your body needs.
Find out more about excessive sweating at WebMD.

Antiperspirants act by plugging the sweat duct, thus reducing wetness. Because perspiration stimulates odor-causing bacteria, antiperspirant also helps reduce body odor. See this explanation from WebMD on Active Ingredients in Antiperspirants.

Perspiration itself is essentially odorless and evaporates readily. However, in areas like underarms, where perspiration can't readily evaporate, it causes the feeling of wetness. Bacteria, which thrive in warm and moist surroundings, break down the perspiration and create underarm odor.

Unlike antiperspirants, deodorants don’t reduce wetness; they help reduce the bacteria that thrive in perspiration and cause the odor. By doing this, deodorants cover up underarm odor. Additionally, most deodorants are scented to also fight odor.

Deodorants help control odor primarily by masking the underarm odor caused by the bacteria interacting with perspiration and also by reducing odor-causing bacteria. It’s actually these bacteria, not the wetness, which causes the odor. Antiperspirants help control wetness, and thereby odor, by slowing the flow of perspiration to the surface of the skin.