The XLR connector is an electrical connector design used mostly in professional audio and video electronics cabling applications, for microphones and line level signals. The most common is the three-pin XLR3, used almost universally as a balanced audio connector for high quality microphones and connections between equipment.
XLR connectors are superior to many other audio connectors for many reasons. Balanced XLR connectors use large diameter signal pins. They feature apositive locking action and incorporate properly designed strain relief as a feature. The female XLR connectors are designed to first connect pin 1 (the earth pin), before the other pins make contact, when a male XLR connector is inserted. With the ground connection established before the signal lines are connected, the insertion (and removal) of XLR connectors in live equipment is possible without picking up external signals. XLR connectors have rugged metal shell that can withstand the hard field enviroment without damaging (for example someone walking over the connector).
XLR connector is always better than other less robust audio connectors, but in the world of XLR connectors there are just acceptable quality and very good connectors. Many cheap and older XLR connectors look like this:
The downside of this design is that you have several small screws on the case. When you need to install the connector you need to open all of them and close when you have done the work. Repairing this kind of connector at the field is thus troublesome (you need suitable screwdriver with you and you easily loose the small screws). Also the strain relief system on the connector is not the best, especially it has hard time to securely hold the thinner cables.
Neutrik started quite many years ago to make the following kind of improved XLR connectors:
Their improvemens include the fact that you can easily open and close the connector without any tools or handling screws. This makes usign the connectors and repairing cables on the field easier. I really like this connector design. I think is is a huge improvement over the older design and worth of some extra price.
Schulzkabel also makes quite similar looking connectors that are cheaper. I have successfully used them and they feel like almost as good as the Neutrik XLR connectors.