XHTML-MP is an extension of XHTML Basic 1.0, a subset of XHTML 1.0 defined by W3C and originally also intended for mobile devices.
XHTML Mobile Profile 1.0 (XHTML-MP, defined by the OMA) adds extra usability and graphical features specific to the mobile space, through the introduction of WAP CSS (WCSS [PDF]) and the style tag and attribute. Since XHTML Basic is a subset of XHTML-MP, the majority of WAP 2 browsers can also manage XHTML Basic content.
See also What's with the HTTP server MIME types? If you want an overview of all the markups related to the mobile, check out this great article originally from Barbara Ballard, What is the difference between XHTML MP, XHTML Basic, WML, i-mode, and HDML?