What is difference between ball valve,gate valve & butterfly valve

also i want to know their application areas

Ball valves, as the name implies, are stop valves that use a ball to stop or start the flow of fluid.

A gate valve, also known as a sluice valve, is a valve that opens by lifting a round or rectangular gate/wedge out of the path of the fluid. The Gate valves are used when a straight-line flow of fluid and minimum restriction is desired.

A butterfly valve is a valve which can be used for isolating or regulating flow These valves can be used effectively in freshwater, saltwater, lube oil, and chill water systems aboard ship. The butterfly valve is light in weight, relatively small, relatively quick-acting, provides positive shut-off, and can be used for throttling.
The butterfly valve has a body, a resilient seat, a butterfly disk, a stem, packing, a notched positioning plate, and a handle.

A butterfly is a vale that has a handle that looks like a butterflys wings you twist to open and twist to close you can google image them too

Wikipedia can provide the answers
http://en.wikipedia.org/wiki/Valve

watch my spouse stumbled on