XOXO
1. Short for eXtensible Open XHTML Outlines, XOXO is an XHTML-friendly outline format for processing by XML engines, where an outline is a hierarchical, ordered list of elements.
The XOXO specification can be found at http://microformats.org/wiki/xoxo
2. Short for hugs and kisses, XOXO is a way of letting another you care and are sending them lots of hugs and kisses. Many people will also extend this beyond XOXO by adding additional X's and O's or use both upper and lowercase. Both the X and the O can be thought of as a hug and kiss therefore it is acceptable to say the X is a hug and the O is a kiss or vice versa.
Also see: Chat definitions, Programming definitions, XHTML