FOAF is an RDF format intended to provide "a way of representing information about people in a way that is easily processed, merged and aggregated" and is primarily concerned with allowing an author to provide a detailed personal description, as well as provide machine-readable links to, and information about, other people. Technically speaking, FOAF is simply an RDF vocabulary. Its typical use is akin to that of RSS: You create one or more FOAF files on your Web server and share the URLs so software can use the information inside the file. Like creating your own Web pages, the creation of your FOAF data is decentralized and within your control. An example application that uses these files might be a community directory where members maintain their own records. However, as with RSS, the really interesting parts of FOAF come into play when the data is aggregated and can then be explored and cross-linked.
There is also XFN : a lightweight method of annotating links to indicate a personal relationship with the person responsible for the linked resource, and as such strengthens existing links in a manner that is both machine-readable and human-comprehensible.
Technically speaking, XFN is a relationship datatype with suggested values provided for ease of use in a wide variety of situations, including HTML, XHTML, and XML documents. Thanks to its simplicity and its foundation in existing specifications, it is very easily understood, and it is easily adopted by authors. By building upon the links that already exist in the Web, XFN complements existing information in place, as it were.