A common variety of pool valve found on pool plumbing. Relatively inexpensive to purchase, and simple to use. Good replacement valve for gate valves.
The disadvantage to these valves is that they are not serviceable, that is, they cannot be opened up to make repairs or clear clogs from the valve. The handles on ball valves tend to break rather easily, and they can also be somewhat difficult to turn, especially if installed in an awkward position.
Their advantage is their cost ($10 - $15), and ball valves tend to be easier to understand for novice pool operators than the three way type valve.
Ball Valves for sale >>here<<