A word clock is like a metronome for digital audio. Every piece of digital gear has an internal word clock. The clock tells the device when to take a sample of the audio. If your sampling rate is 48kHz, the word clock "beats" at 48,000 times a second. Some devices have better work clocks than other. What separates a good clock from a bad clock? Better clocks have a steadier "beat". They are more stable.

When sending digital signals between digital devices, each device must be sampling at the exact same time as the other devices. If they aren't, you get digital clicks and pops. Whenever you are using several digital devices, you must specify which device is providing word clock. In the past, the device providing the clock would be called the Master, and the devices getting their clock from the Master were called Slaves. There is a movement to get away from that nomenclature, now calling one device the Primary, and the other devices Secondary.

A stand alone wordclock generator provides a much more stable clock than what manufacturers are putting into their devices. When using a device like the Big Ben, it is designated as the Primary, all other devices are set as Secondary.

EDIT: 'amoeba' gave the following explanation at the Harmony Central discussion forum where the same question was posted:

'A word clock is the timing signal that tells the converters in your system to take a sample and either read it in or spit it out. It sets the sample rate e.g. 44.1kHz, 96kHz etc...
A system usually has them built in, but better units have the option of running things from an external clock, so that you can have one clock tying everything in your system together.
A more expensive clock can also be more accurate than the built in one. Slight variations in timing can change the signal, most people talk about 'tighter definition and imaging' from a better clock.
Upgrading to an external word clock unit is fairly high end polish kind of stuff, you are probably better off looking at things that will make a bigger difference to your sound first, such as your preamps, microphones and monitoring.'

Nice answers folks!

What you won't hear the manufacturers of wordclocks tell you, though, is that there is a very big technical/theoretical DISADVANTAGE to running an external wordclock, as far as jitter goes.
The problem is the PLL chips. Pretty much any audio interface device with more than 2 channels has one in it, and a wordclock generator has at least one in it as well. All PLL chips generate a certain amount of jitter; they smooth out and virtually eliminate incoming jitter from the clock source, but then they introduce their own to the clock output.
In the typical audio device, the wordclock is designed thusly:

Crystal Reference->PLL chip->Converters

When you get an external wordclock unit, all you're doing is replacing the first part of the chain: the input reference to the PLL. The PLL in the audio interface will still introduce its own jitter, and in many units (MOTU, M-Audio, etc), this jitter is around 250-500ps.
Yeah, a lot of interfaces come with cheaper, less accurate crystals in them; this is where an external wordclock will make an audible improvement. However, a more accurate crystal can usually be DIY'd into the unit for less than $20 - way less than the price of an external clock. In some cases, you can even replace a device's PLL chip with a better version for lower jitter (they're typically under a dollar). This would provide a bigger improvement than any external clock.

The main advantage of having an external clock is if you have multiple digital devices you need to sync up. In this case, good jitter accuracy is important, even if it's not necessarily better than your unit's internal clock. Most companies advertise the ability to sync up all your devices accurately as the main selling point of their wordclock. The community ignores this and claims massive audible improvements on their one audio device just by having a particular external clock.
Are they right? Partially... you CAN hear subtle or even slightly-obvious differences, particularily if you have a cheaper audio interface with a cheap crystal. Is it a good-value upgrade? Not at all. A good converter with a good internal clock has a strong electrical advantage, and it's way cheaper for what you get.

Cedar Flat Fats wrote: Nice answers folks!

Kurt,

Before giving Ellegaard praise for his answer, you may want to talk a stroll over here:
http://acapella.harmony-central.com/forums/showthread.php?s=&postid=8721658#post8721658

Apparently, the original poster posed this same question on several differnt bulletin boards. Ellegaard decided to take advantage of this by lifting a response from the Harmony Central board and posting it here. Then he lifted my response here, and posted it over on Harmony Central.

Pretty lame, if you ask me.

D'oh, I screwed up, again! I must have been somewhat tired yesterday when I surfed around and browsed the different groups. I noticed the same question on both groups, and somehow I found it amusing to cross-post the two (by the way excellent) answers to the question. Believe it or not, it didn't cross my mind that it could be taken as an attempt to get credit for other people's posts, which wouldn't make sense. Neither did I even think about the consequences. My bad, I owe Erick and Amoeba an apology. I have now credited both of you correctly.

I was credited as the idiot! :-( I'll try to behave for the future!

Ellegaard,

Like I told you on the Harmony Board, it's all good. I was just a little shocked to see my words coming out of someone else's mouth. I trust you meant no harm.

Take Care.
8-)

a wordclock is like a normal clock, but instead of numbers, it has words..
for example: where a normal clock would say '8', a wordclock would say: 'eight'..
...