On headphones three pins are used for stereo and two pins for mono, yet on microphones three pins are often used for mono; why? Also why is a oddly-shaped plug (XLR) used rather than a jack?
XLR is designed to carry a "balanced" signal. Pin 1 is ground, Pin 2 is "in phase" or "+" and Pin 3 is "out of phase" or "-". See for example http://www.clarkwire.com/pinoutxlrbalanced.htm
The reason is the following: The microphone signals are very low so they are vulnerable to noise that gets injected into the cable especially if the run is long. Noise gets typically injected into both signals the same way. The receiver than forms the difference between the + and the - signal. That cancels the noise. Since the "-" signal is out of phase, the difference is actually twice the desired signal.
The"balanced" approach also helps with avoiding ground loops in larger systems. The actual signal is always the difference between "+" and "-" and therefore no ground is needed.
The choice of male vs female is guide by "no exposed pins should go into the amplifier". Pins can be touched which can create a lot of noise, pops or even damage a loudspeaker. Hence inputs are female and outputs are male.
Signal/ground/shield are the three leads to a mono microphone. Or +/-/sheild. On other occasion there will be phantom power to the microphone to turn it on or to provide voltage to active electronics. Etc... Etc... The fact is that your microphone being mono is not the reason why it has three leads it can operate with two. There are three leads because the cable is shielded or insulated against interference. The 1/4 inch phono plug compatable with this is "tip-ring-sleeve" however more popular and standard for microphones is "xlr" the industry standard.