I2S

The I2S Audio Interface provides a synchronous serial interface to off-chip audio devices.

The I2S or Inter-IC Sound Standard was developed by Philips to allow audio
data exchange between converters, filters and digital input/output interfaces.
The serial interface is a three-wire bus that contains a line for two time multiplexed
data channels (Data), a word select line (L/R clock), and a clock
line (Bit Clock).

The standard also dictates that the actual audio data be delayed by one Bit Clock period from the L/R clock.

This is the protocol used inside CD players to connect the transport and the DAC.

The major difference with SPDIF is that both sender and receiver use the same clock.

This eliminates input jitter.

Connecting

I2S is designed as a protocol between chips on the same board.
Some manufacturers use it to connect boxes.

As there is no standard for cabling, each does it in his own way.

Even if both boxes have the same connector, if they are from different brands, the pin layout will be different too.

Some of us use I2S externally too, with good results. Like S/PDIF though, the devil is in the detials. The Transmission-lines must be terminated and the edge-rates must be fast in order to avoid adding significant jitter. Really low-loss high-bandwidth cables are required.

Audio Asylum

Measurements at ASR showing the jitter performance of I2S and SPDIF.
Almost no differences and at such a low level, it won’t be audible at all.

Bottom line, please don't rely on fantasies like I²S improving audio performance. Insist on measurements being provided to demonstrate audible change in the output of the DAC -- the thing you hear. Don't accept hand gestures as evidence. And certainly not any appeal to a lay person's intuition as a replacement for proper data providing proof.

Study: Is I²S interface better for DACs than S/PDIF or USB?

References

I2S bus specification - Philips Semiconductors
Inter-IC Sound Bus - Interfacebus.com
DIGITabilis: crash course on digital audio interfaces - TNT-Audio
I²S - Wikipedia
Study: Is I²S interface better for DACs than S/PDIF or USB? - ASR

The Well-Tempered Computer

I2S

Connecting