Taken from AMD's website.
You expect reliability and stability from your server solution and you get just that with servers powered by the award-winning AMD Athlon™ MP processor. Built on AMD's stable Socket A platform, the AMD Athlon MP processor for servers is designed for an easy and flexible upgrade path to address a wide range of commercial computing needs.
AMD's innovative Smart MP technology uses dual, independent point-to-point system buses to increase available bus bandwidth. Along with an exemplary cache management system, Smart MP technology allows for high-speed communications between processors, helps reduce data transfer latencies, and helps ensure that both processors work to their full potential.
When it comes to compatibility, the AMD Athlon MP processor is designed for exceptional performance on the world's most popular server applications. We've worked with leading vendors and have designed our processors to be compatible with more than 60,000 applications and counting. In fact, more small and medium business and enterprise users discover the benefits of the AMD Athlon MP processor every day.
Basically, MP is AMD's server grade CPU supporting multiable processor (mp?). Well that and maybe about $50+ difference in cost.