If you’ve been playing with the Sandbox theme‘s code, you will notice that within the theme’s sidebar files you get to see lists classed as “xoxo”. Even more curious, WordPress’s Template Tag wp_list_bookmarks also adds this class by default to its output.
So what is XOXO?
XOXO is a simple, open outline format written in standard XHTML and suitable for embedding in (X)HTML, Atom, RSS, and arbitrary XML. XOXO is one of several microformat open standards. (Source)
So basically it’s one of the microformats specifically used to display lists and outlines. The purpose of it, as is the purpose of microformats in general, is to be
a set of simple open data format standards that many are actively developing and implementing for more/better structured blogging and web microcontent publishing in general (About Microformats).
Now there you have it.