Topic: Two Different Tag Cloud Widgets
Notice 2 different Tag Cloud widgets?
The "ST Tag Cloud" widget is a plugin I've added and has extra options to administer. And, at present, it can be cloned into multiple copies of itself at the bottom of the widget page(I don't know why I'd ever need several different tag clouds!).
The "Tag Cloud" widget is the built in widget. Compare what each does on the front page with a variety of themes.
BTW, wikipedia explains nicely what tags are, and why the feed my inner geek.