XLR connectors, also known as cannon connectors, are an industry-standard professional range of connectors. They are mostly used for audio signals but can be used in other applications such as power cables.
XLR connectors are also available as 2-pin and multi-pin versions with many pins.
XLR connectors are often referred to as Cannon connectors after the company that originally developed the design. Note that this is not the well-known Canon brand, but a different company which is now a division of ITT Corporation.
Cannon's original design was the "X series", which later incorporated a latch (L) and then rubber insulation (R), leading to the acronym "XLR".