XOOPS has evolved from several products. First there was ThatWare, a Slash clone written in PHP. Then PHPNuke came along and was written on top of ThatWare. PHPNuke was written in PHP and used MySQL for a database.
For various reasons, many of the PHPNuke developers decided to create a new version of PHPNuke and named it PostNuke; one of the reasons stated was that the developers wanted to address the security concerns of PHPNuke. However, some criticisms of PostNuke are that the developers change the APIs too frequently and that PostNuke is too heavy on computer processing power. PostNuke has in turn created a new branch, first called LostNuke and then Xaraya, created by some of the key developers of PostNuke.
XOOPS appears to be an object-oriented rewrite of PHPNuke. The XOOPS development teams believe that a system that uses objects is more secure and easier to develop and maintain.
More detailed information about upcoming XOOPS releases and the roadmap for XOOPS is available at http://devteam.xoops.org.