Symptoms of Chafing:
The most common symptom of chafing is a painful stinging or burning sensation. The area is usually red and raw.
Cause of Chafing:
Chafing is caused by repeated motion -- specifically, skin rubbing against loose fabric or other skin. Chafing most often occurs around the bra line (women), nipples (men), inner thighs, and under the arms. Moisture, either from sweat or rain, can worsen chafing. It can also be caused by a poorly fitted bra.
Prevention of Chafing:
Before long runs (you don't have to worry too much about chafing during your shorter runs), spread a thin layer of BodyGlide or Vaseline on vulnerable areas.
To prevent chafing, wear running attire made of synthetic materials such as CoolMax that wick moisture away. Don't wear cotton clothing because once it gets wet, it stays wet. In addition, cotton is a rough material and when it's constantly moving against your skin, it can rub your skin raw. (If you've ever seen a male runner with bloody nipples, he's most likely wearing a cotton shirt.)
For women, make sure you're wearing a synthetic sports bra with smooth seams.
If you're doing a long distance race such as a marathon, you should assume you could chafe, even if it's never happened to you before. Use Body Glide or other lubricant and make sure you race in clothes and gear that you've tried out during your long runs.
Proper hydration is also important to preventing chafing. Make sure you follow steps to stay hydrated during your runs.
Since chafing can be caused by loose running clothing, it's better to wear running clothes that are snug. Some runners prefer to wear spandex bike shorts to prevent chafing between their legs.
Treatment for Chafing:
Apply some A+D ointment to the dry/chafed areas -- it should clear up in about a day. Be sure to cover up affected areas with BodyGlide or Vaseline before you head out for a run.
More: Products to Prevent and Treat Chafing