What is Word Clock?

This is a question I get a lot. Word clock is one of the most commonly misunderstood and enigmatic topics in the studio. Here's my explanation...
Every digital device, from a simple portable CD player to a Pro Tools HD rig, has a word clock. Not to be confused with sync clock (like MTC or SMPTE), word clock is what tells a A/D or D/A converter when to take the sample. It fires an impulse to the converter 44.1k (or 48k, or 96k, etc) times per second. The reliability of this clock, how evenly spaced those pulses are, determines the accuracy of the conversion process.
If you have digital connections in your studio (S/PDif, lightpipe, etc), you MUST have a single clock source. If multiple devices are each trying to be the master clock, you will probably experience pops, clicks, or sometimes chirps in your audio. The solution is usually to make sure that you only have one master clock, and that all slave units are getting a clean clocking signal.
But even without obvious problems like pops and clicks, you may still have more subtle word clock problems. Think of it like this...
Ever seen clips from the very early days of film? Back in the days of hand-cranked cameras, it was extremely difficult to get the shutter speed to be perfectly even. The result was a moving image that, when played back at a constant shutter speed, seemed "jittery." the motion was often jerky an unnatural. This was due to the imperfections of the original film speed.
The same thing happens in digital audio. The more "jitter" present in the clock source, the less accurate the resulting conversion will be. Jitter is manifested in audio as phase distortions and discepancies, particularly in the higher frequencies. They can lose some of their sparkle, transients can lose some attack, and stereo images aren't as vivid.
Improving the quality of your clock is an easy way to upgrade every A/D and D/A converter in your studio. In most cases, adding a high-quality master clock, such as Apogee's Big Ben or Antelope Audio's Isochrone OCX will result in a marked improvement in the overall quality and clarity of your finished product.
But what if you don't have a master clock? One of the hallmarks of Apogee and other high-quality converters is usually a low-jitter clock. If you have a great converter in your system, you can feed that clock source to everything else with an A/D or D/A converter on it (like an audio interface, for example). Replacing the built-in clock of your interface with the clock signal from the master clock will dramatically improve the quality of the conversion being performed by the interface. There are still other factors at play, so it's not identical to running the same signal through the high-quality converter, but it is usually a very noticeable improvement.
For more on this topic, check out my friend Nika Aldrich's book Digital Audio Explained for the Audio Engineer, available through Sweetwater.

A word clock or wordclock is a clock signal used to synchronise other devices, such as digital audio tape machines and compact disc players, which interconnect via digital audio. S/PDIF, AES/EBU, ADAT, TDIF and other formats use a word clock. Various audio over Ethernet protocols use broadcast packets for the word clock. The device which maintains the word clock on a network is the master clock.
Word clock should not be confused with Time Code; word clock is used entirely to keep a perfectly-timed and constant bitrate to avoid data errors. The word clock generator, usually built-in to analog-to-digital converters, creates digital pulses which contain no other data, and is considered essential to avoid frequency drift between the internal oscillators of each device. Timecode is actual data (technically metadata) about the media content being transmitted, and is optional, being sent in a higher layer.
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Thank you, Wikipedia. You've just been banned.

Bought a Mackie Onyx 400F and learned that the 400F has a word clock; this would allow me to add another device while using the word clock source: great . Been using it three years and I love its sound but I never used 400F with another device simultaneously.
Now I want to add a Mackie Onyx 1640i to my kit but 1640i does NOT have a word clock in or out. Should I be worried about it?
Should I avoid using both devices simultaneously?
If I record with all 24 channels (1640i + 400F) does this degrade the audio quality?
Thanks in advance.

I have an RME Fireface UC and I have a manufacturer telling me their product will actually replace the internal clock of my unit with theirs, and to reclock the data stream. I was under the impression that all an external clock did, was to sync all the sources to the same clock, not replace their internal clock.

I have an RME Fireface UC and I have a manufacturer telling me their product will actually replace the internal clock of my unit with theirs, and to reclock the data stream. I was under the impression that all an external clock did, was to sync all the sources to the same clock, not replace their internal clock.

If you're using an external clock then you're replacing the internal clock source with the external one. Six of one, half a dozen of the other.
With a decent piece of hardware like a Fireface it's unlikely an external clock will yield any objective performance improvement. In fact I've heard a convincing argument (from a designer of digital circuits) that it's far more likely to degrade the signal.