This is the kind of 0.25" stereo jack you might find in home stereos and PA systems. Switched contacts allow you to detect whether or not there is something plugged in to the jack. The jack can be panel mounted in an 11mm diameter hole.
The jack comes with a plastic washer and nut for panel mounting.
Note: Looking for a breakout to hook this audio jack up to your project a little easier? Check out the SparkFun TRS Jack Breakout!