XLR cables are probably the most important cables in live sound. They are used to connect microphones, among other things, to mixing desks, audio interfaces and speakers.
Many people will refer to them as "microphone cables". They are BALANCED connections, meaning they will not receive interference over distances. XLR cables have three pins on the male end and three holes on the female end.
A thing to remember is the pins point in the direction the signal travels. So any output (e.g the bottom of a microphone or mixer master out) will be male, while the inputs of a mixer are female. Because each end is opposite, you can plug an XLR cable into an XLR cable and "daisy chain" them to make a longer cable with no side effects.
The benefit of XLRs is the fact that they clip in and so will not disconnect with tugging. As long as someone doesn't pull so hard that they pull the mixer off the table or the speakers down, then you should be fine (this has not happened to me yet at time of writing).
These cables are quite bulky and heavy as well as moderately expensive so consider your needs before buying a 15m cable to connect your microphone to the audio interface right next to it.
Because it has a balanced connection (a pin for ground, hot and, cold), each XLR cable will only carry a mono signal so you will need two if you are doing a stereo out from the master mixing desk.
For more posts in the series of Cables and What They Do, click below: