XLR connectors are most often found on microphone cables, and are the most widely used connector to transfer balanced audio. Typically 3-pin and often referred to as a cannon plug. As well as being used for microphones, XLR connectors are used on most pro audio equipment for the transfer of balanced line level signals.
They are popular due to their secure locking mechanism and robust design. 3 pins allow for the transfer of two signals and a ground along the one cable. This is essential for balanced audio. Typically pins 2 & 3 carry the hot and cold signals, while pin 1 carries the grounding signal.