MP and Xp

What's the difference between Athlon Xp and Mp.

I'm not sure about the XP but I pretty sure that MP is short for multi processor.
These are used on motherboards that are capable of running two cpus..

Yup, basically the MPs are designed for multiprocessing, while XPs are for use on single processor boards. MP's also come unlocked while the Palomino XP's don't (however MP's aren't as good overclockers). You can make a XP into an MP by bridging the last L5 bridge (you don't even have to fill the gap, just go around it). Doing this will make the mobo think it's an MP and then you can use two XPs in a MP motherboard. If you need to know more, there's info about MPs in the stickies in the SMP section.