As been said over this is a big internal (architectural) change for Apple, but for a normal user, i.e. not software developer, the switch should (hopefully) not be noticeable at all. All your favorite Apple and major 3rd party applications will run as they always have.
You might even gain a feature, with the ability (through a framework called WINE) to run any applications written for Windows, but that's speculation for now.
Apple has done this because first Motorola and now IBM has not been able to develop and produce faster PowerPC processors, so the advantage this type of processors originally had (at same or even lower clock speeds) have been eaten up by Intel's development of faster chips with improved architecture and lower power consumption.
So, even if the G5 is an awesome desktop computer right now, Apple does not believe IBM will be able to deliver good enough chops in the future (and Apple has been burned by IBM's promise to deliver 3.0 GHz chips by summer of 2004, now a year later the fastest chips are running at 2.7 and Microsoft's XBOX2 (or 360 or whatever) has been promised most of the production of chips at 3.2 GHz which should be out later this year.)
And then there's the issue with laptops, the G5 has proven impossible to fit into a laptop and the G4 is limited by a very slow BUS (which is how the processor speaks with the rest of the hardware). So Apple's laptop lines has, effectively, not seen any real improvement in years and had no prospects to do so. So moving to a Intel Pentium M derivative just made sense, from that point of view.
Now, it just remains to be seen if Intel manages to build in enough security in the hardware and Apple to continue to keep OS X free from exploits, if they manage that, you, as an end user probably wont notice if you have an Intel or PPC Mac (except the Intel one probably will be faster...
)
