P4 compared to P4M is simply a case of the P4M having some additional power-saving features, as well as being tested and verified to run at lower voltages. Clock for clock, the P4 and P4M should be essentially identical (at least, assuming that the chipset, cache, FSB, etc. is clocked the same), but the P4M should be able to consume less power. So a regular P4 2.4 would run at 1.5V, I believe (actually, according to [support.intel.com...] it could be 1.50V or 1.525V, and the P4 2.4C might have other options), whereas the P4M would be a 1.30V part at the same speed (http://support.intel.com/support/processors/mobile/pentium4/sb/CS-007499.htm), and it would scale down the voltage level as the processor ran at lower speeds. Finally, if we toss the Pentium-M back into the story, it runs at 1.484V for the top speed of 1.6 and 1.7 GHz, and uses lower voltages as it scales down speed. For typical power dissipation, the desktop P4 2.4 is around 60W (66W for the 800FSB version), the 2.4 P4M is 30W, and the 1.6/1.7 P-M is 24.5W.
Note that the P4M is only available in 100 MHz (400FSB) bus speeds, whereas the desktop version is available in 100, 133, and 200 MHz speeds (2.4, 2.4B, and 2.4C, respectively). With its dependence on memory bandwidth, theP4 2.4C is a LOT faster than the original 2.4 and the 2.4 P4M, at least in most applications. When in a laptop, the use of slower hard drives, memory, graphics, power saving features, etc. all combine to make a 2.4 GHz P4/P4M laptop substantially slower than a 2.4 GHz desktop. Throw the P-M into the mix, and things change even more. The P4 is designed to benefit from high clock speeds, but high clock speeds mean high power requirements. The P-M is designed more like the Athlon chips: higher efficiency, lower clock speeds. In addition, it is designed to rapidly cycle power to areas of the CPU that are not needed. It does this for the cache as well. All told, the P-M is a really good mobile solution. (And it has pretty much succeeded in killing off the Transmeta CPUs when it comes to laptops.)