XLR

For XLR as the connector with variants, agnostic to use, see Common_plugs_and_connectors#XLR

For XLR3 used for balanced audio, see balanced audio as a concept,

(a now-rarer use in audio is digital audio, specifically the older AES/EBU)

For "I am handling things on stage and I have a question"

for more practical questions, see Music - studio and stage notes - cables, connectors, adapters

for more technical questions, perhaps

The XLR pin 1 problem

audio voltages and impedances

For XLR5 and XLR3 used for DMX, see Electronics_notes/Localish_communication_notes#DMX_cabling