A microphone (Mic) is a device that turns sound into electrical signals. More specifically, it transforms sound that it receives through the changes in air pressure that impinge upon it. These changes result from sound in the environment and so microphones can be made directional - picking up sound from where they are pointed or omnidirectional - picking up sound from all around.
There are two kinds of Mic. One requires no power and the other does. The former uses materials that naturally produce electricity (albeit in minute amounts) when moved such as a magnet in a coil of wire. These are called Dynamic Mics and are either moving coil or ribbon and are often used on stage, being less expensive than the other kind and more robust. Another type of mic that does not require power is the piezoelectric. This uses a crystal which generates electricity when subjected to pressure. and is used as built-in the mic on portable equipment and also forms the basis of many instrument pick-ups. This type is also often used in Electronic Tuners.
The other type of microphone uses batteries or a dc electrical supply (typically 12 to 48v) from the equipment to power a device which will provide the translation of sound into signal. The latter usually provide more output but do require powering and are less robust than dynamic mics. The equipment-provided power is called a Phantom Power Supply. These mics are Condenser (or capacitor) Microphones and are preferred in studios for recording but are often used on stage.
Wireless microphones are simply ordinary mics (any of the above) with a radio to transmit the signal to the equipment rather than use a wire. A base station receiver translates the radio signal back to an electrical signal.
The signal from a microphone is usually fed into a Mixing Desk before being fed to an Amplifier.
Different Mics have different characteristics and are designed for voice or instruments for example. Mics can be made to style and fit into soundholes or instrument bells. They can be made to pick up instruments from Harmonica to Drum.
Names that have been long associated with Microphone manufacture are Shure, Samson, AKG, Sennheiser, Rode and Audio-Technica. The Shure SM58 Vocal Dynamic Microphone for example is a legend and is found in the kit of just about every band.