"A "public suffix" is one under which Internet users can directly register names. Some examples of public suffixes are .com, .co.uk and pvt.k12.wy.us. The Public Suffix List is a list of all known public suffixes. "
I saw this as a MetaFilter question once. That thread did more to expand my reading list than any single page. Pretty nice to see it (or something like it) distilled into a site.