A World Clock used to be something our fathers used to put in their home offices or dens or home studios, which was basically a map of the world showing all the time zones, with a scattering of little digilog clocks located at many of the major cities of the world on the map, showing the local times there. Thay usually looked something like this :
and did nothing to help the sound of anything
A word clock OTOH, as stated, is basically a digital time signal sent over wire to the various digital devices in the studio to help ensure that they all synch up and "fire" at the same time, rather like the timing chain on an old automobile engine. In this case, though, that means that whatever sample rate your gear is working at, that they all take their samples at the same time so that the 5-second point in your recording is always exactly represented at the digital sample representing 5.0000 seconds and not the one representing 4.9999 seconds or 5.0001 seconds (numbers made up for illustration) in every piece of digital gear you have. Such discrepencies are called "jitter" (among other things) and can cause some small amounts of distortion in the signal.
There is a lot of controversy as to how much this really matters on an audible level and whether word clocking actually helps reduce jitter or not, but basically Ethan is right when he says it's an issue that's way down on the list of concerns for the home recorder, because it's such a fine difference. We all have a lot more bigger issues by orders of magnitude to worry about with our sound before we need to worry much about that. But for the more experienced users with better gear who have already ironed out most of their other concerns, it's worthy not to ignore.
G.
