2.5mm, 3.5mm and 6.3mm jack connectors
Jack connectors come in three sizes (2.5mm, 3.5mm and 6.3mm) and are available in both MONO & STEREO versions

2.5mm & 3.5mm jack connectors
These are both smaller versions of their big 6.3mm brother and are most often used as audio connectors for personal audio equipment.

6.3mm jack connectors
 
There are two versions of this connector in common use, MONO & STEREO. The stereo plug is used for balanced operation and wired as follows:

TIP - Signal +
RING - Signal -
SLEEVE - Screen
For unbalanced operation the RING & SLEEVE should be joined.

The MONO version can only be wired unbalanced as follows :

TIP - Signal +
SLEEVE - Screen