According to Wikipedia, "Cavitation is the formation of vapor cavities in a liquid, small liquid-free zones ("bubbles" or "voids"), that are the consequence of forces acting upon the liquid. It usually occurs when a liquid is subjected to rapid changes of pressure that cause the formation of cavities in the liquid where the pressure is relatively low. When subjected to higher pressure, the voids implode and can generate an intense shock wave."
Cavitation presents serious concerns to engineers and specifiers of control valves. Understanding ways to mitigate cavitation is important. Flowserve has put together this excellent document on understanding cavitation and offers solutions on reducing or eliminating its effects.