Because I'm married to a typographer, I suppose I'm a bit more interested in typography than most writers.
Tag Clouds seem to be cropping up everywhere nowadays. The link I just gave you is wikipedia's take on them, and of course will be much more accurate and insightful than anything I'm about to write here.
But essentially, tag clouds show the frequency/reach with which words, or tags, are used in search or links.
(I wonder how long it will be before an Art Director hijacks the look and feel of tag clouds for print or TV ad graphics.)
Most tag clouds seem to be based on system fonts right now, but hopefully soon, some enterprising designer/typographer/geek will figure out a way to combine any font with the usage data/point size of the tag. Maybe even based on vector graphics to keep the integrity of the line.
I now realise I'm not only rambling, but straying way off line, and into areas I know absolutely nothing about it.
So I'll end this post.
ps. Sorry, I can't shut up about them.
Here's a particulalry fine example of a tag cloud, from a particularly fine and inspirational blogger; Planner, Russell Davies.