FOAF is a way of specifying personal information about a person, including nicknames, links to homepages and lists of people they know, in a XML document. The result is machine-readable "semantic web" homepages which can be traversed, forming social networks; in other words, something like
Friendster, only open, XML-based and without the emphasis on bootywhang (though surely someone will come up with a namespace for that in due time), and traversable by any of the countless FOAF browsers due to be written in Perl, Python and Java any day now, or by the
web-based FOAF Explorer; if your browser does SVG, there's even a graphical network browser there you can use.
So how do you make yourself a FOAF document? You can get started with the FOAF-A-Matic (Javascript required).
(thanks, cos)