You would have seen people using XOXO at the end of a written letter or email. The sequence of letters XOXO indicates Hugs and Kisses, which is typically used to express affection or good friendship [wiki]. There is another XOXO in the technical world, which is relatively new.
XOXO – eXtensible Open XHTML Outlines is an XML microformat for outlines built on top of XHTML. It is an attempt to reuse XHTML building blocks rather than inventing unnecessary new XML elements/attributes. XOXO is based on existing conventions for publishing outlines, lists, and blogrolls on the Web. The XOXO specification defines an outline as a hierarchical, ordered list of arbitrary elements. [more at wiki]