"What's the differences between G4/G5/AltiVec and Intel based Macs?"

Most common question so far I've heard.

The short answer is this:
In a nutshell, there are no differences in between them from a user perspective. You should not be able to see a difference except in improvements of speed. Apple did a great job migrating OS X to Intel processors without a single hitch, and problems are not really obvious to the users.

The long answer:
The differences in the processors (Intel & PowerPC G4 / G5s) are actually worlds apart. However, both software written for either G4/5 or Intel will work and run well on your Mac, and provided that the application is Universal, both your Intel and PowerPC Mac will run and read these apps and their files just fine. Older PowerPC apps run on an Intel Mac through Rosetta.

Previously, it was thought that switching processor platforms would be an almost impossible task as they run on completely different instruction sets.

If your first Mac is an Intel one, then you shouldn't worry anything at all about the differences at all. It will run older software and most newer software are Universal Binary or meant for Intel.

As for why Apple decided to switch to Intel processors - there are many reasons - but in my opinion all of them has to do with more power, flexibility and availability. (The Intel's P4 processors actually aren't really good until the Core Duos came out.)

I'll add more in time. Should you have any questions you may post them here too.