Stereo output jack sockets are essentially a standard mono jack, with an additional ground connection. This extra ground connection is typically used as part of the power circuit (usually 9v) to power onboard effects and active pickups in your guitar. The reason for having the circuit through the jack is so that when the cable is pulled out of the guitar, the circuit is broken, stopping your batteries from continuing to drain when your guitar is not in use.
Stereo output jack sockets are a must for anyone who uses any powered devices on a guitar - otherwise you could find yourself turning up to a gig with a dead battery and a silent guitar!
Dimensions -
Thread length: 7mm
Thread width: 9mm
Total length: 32mm
Maximum diameter (at solder lugs): 25mm