Get an impression on XFN [1] first:
XFN is 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.
XFN in combination with CSS2 enables a very easy way to viszalize annotations. XFN is much easier to parse and handle and from that point of view, I am sure it will spread faster in conjunction with blogs as FOAF currently is doing. It is no replacement of FOAF but a gateway drug ;-p
[1] http://gmpg.org/xfn/
