Probably the most common professional audio connector is the 3-pin XLR connector. These are often referred as Cannon connectors, after their original manufacturer; even though today they are also made by Switchcraft, Neutrik and others.

3-pin XLR connectors are used for balanced, analogue audio circuits -- both microphone and line level.

• Pin 1 is the ground pin and protrudes past Pins 2 and 3 in the female connector. When inserting the male; the ground pin makes contact first. This helps to avoid annoying buzzes and pops when connecting live equipment.
  

• Pin 2 carries the "HOT" or "in-phase" signal.

• Pin 3 carries the "LOW" or "out-of-phase" signal.

3-pin XLR connectors are also used for AES/EBU professional digital audio interconnections.

Clearcom "party-line" intercoms use 3-pin XLRs for interconnection between belt-packs.

4-pin XLR connectors are used for the headset on Clearcom intercoms.

5-pin XLRs are used for control signals for various pieces of lighting equipment (DMX -- Digital MultipleX).

A mini, 4-pin version of the XLR is used as the input connector on wireless microphone body packs.

The Tip-Ring-Sleeve (TRS) or Quarter-inch phone plug  is often used in place of  an XLR connector on audio equipment.  The TRS is  less expensive and takes up less space on the front or rear panel.

Tip-Ring-Sleeve (TRS), Stereo or dual-circuit phone plugs are used for:

• balanced inputs and outputs on audio equipment, like consoles, equalisers or power amps.
• TIP --  corresponds to Pin 2 of the 3-pin XLR connector;
• RING --  to Pin 3
• SLEEVE --  to Pin 1 (ground).
• stereo headphones
• TIP -- left earpiece
• RING -- right earpiece
  
• SLEEVE --  common (return).
• Many consoles use a TRS connector for the channel insert. This is a kind  signal "detour" out of the console and back, to allow an external FX processor to be connected inline with the signal path.
  
• TIP -- is used for the send signal
• RING -- for receive
• SLEEVE --  common (return).

Tip-Sleeve (TS), Mono or single-circuit phone plugs are used on consumer and semi-pro equipment.
They are found on:

• microphones
• electric guitars
• some semi-pro processors (e.g. SPX-90, 900)
• passive loudspeakers

RCA plugs, also known as phono plugs are unbalanced connectors used on consumer audio and video equipment. More expensive, high-end models may use nickel or gold-plated versions.

Originally used to connect microphone-level outputs of a phono cartridge to the preamp; they are more commonly used for line-level signals today --

• audio inputs and outputs of audio and video tape players/recorders
• line-level inputs and outputs on computer sound cards
• video in/out connectors on VCRs and cameras
• SPDIF digital audio interconnections on consumer gear.

RCA to RCA cables are a common source of hum caused by so-called ground loops.

The shield of the cable serves as the ground return for the audio or video signal. The chassis at each end of the cable can be at different AC potentials because they are plugged into different AC circuits or mounted in different racks. This can cause 60 Hz AC to flow in the shield. This 60 Hz signal is superimposed on the audio or video signal.

Depending on the gain in the rest of the system, even a few micro-amperes of current can result in an audible hum in a sound system or visible rolling "hum bars" in a video picture.

For this reason, avoid using RCA-RCA cables to interconnect audio or video equipment more than a few feet apart, or in different rooms.  If RCA connectors must be used, make sure all pieces of  interconnected equipment are fed power from the same AC circuit breaker. (same duplex U-ground connector.)

The eighth-inch or mini stereo phone plug is a smaller version of the 1/4 inch stereo phone plug. It is used as:

• headphone connector for laptops, iPods, music players.
• TIP -- left earpiece
• RING --  right earpiece
  
• SLEEVE --  common (return).
• microphone- and line-level input and output connectors on desktop computers.

A 4-contact version of the 1/8 inch mini phone plug is used as the headset connector on mobile phones.

• TIP -- left earpiece
• RING 1 -- right earpiece
• RING 2 -- common (return)
• SLEEVE -- microphone and power.

The RJ45 modular connector is seen more and more often in the theatre where it is used in:

• connecting an Ethernet network to a computer or other digital device.
• connecting an analog (VGA) or digital (HDMI) video source to a video projector or monitor. Usually active or passive adapters are required at both ends of the signal path.
  
• connecting a digital audio console to a digital stagebox, a kind of digital "snake"
• connecting DMX-enabled lighting instruments to a lighting controller.