FOAF or Friend Of A Friend is an XML file format for your personal information, and allows linking to other peoples FOAF files. The below snippet from the FAQ may help describe FOAF better than I can
...provides a way to create machine-readable homepages in the Web. Technically, it is an RDF/XML 'Semantic Web' vocabulary. FOAF documents are most commonly used as a way of representing information about people in a way that is easily processed, merged and aggregated.I created a FOAF for myself and my wife. I created these mainly because with the new site design, we did away with our personal information sections on our blogs, and the concept of FOAF seems like a good one.
FOAF provides conventions for saying the sorts of things that you might say in your homepage ('My name is...', 'I work for ...', 'I'm interested in ...', 'I live near ...', 'I'm pictured in these photos...', 'I write in this weblog...'), but in a way that is easy for computers to process. Since computers are pretty dumb, and can't read human languages, we provide simplistic FOAF descriptions, to help them answer questions such as 'Show me pictures of Weblog authors interested in ... who live near here', 'Show me recent articles written by people at this meeting', 'Is this person vegetarian?'. FOAF is a 'Semantic Web' project, which is an effort to make the Web easier for machines to help us navigate.
For more information on FOAF check out www.foaf-proect.org.