Summary
XLR3
XLR 3-Pin
XLR 3-Pin connector cables ends are an industry standard for microphone and audio signal cables.
Out of the three pin connections, one is for positive signal, one for negative signal and the third is for 'balanced' ground connection to the cable sheilding.
Description:
XLR 3-Pin connector cables ends are an industry standard for microphone and audio signal cables. 
Out of the three pin connections, one is for positive signal, one for negative signal and the third is for 'balanced' ground connection to the cable sheilding.
There is also a locking mechanism in most XLR connector designs that keeps the connections fixed together even when being pulled on or jostled.