CISC was easier and cheaper to develop in the 80's. windows went with that, and windows became very popular. with that, CISC (or x86 compatible processors as they are more commonly referred) became the standard, and not one easily changed.
RISC was more advanced, was more efficient, but ultimately more expensive to develop. IBM was a firm that chose to develop these processors, their POWER line of high-end super-computer processors. the PowerPC was a scaled down version of this, that was suitable for the personal/home computer market. the 3rd generation PowerPC line of chips, apple decided to call G3. the G4 and G5 are the 4th and 5th generations, respectively. Motorola also developed these chips, being mainly responsible for the G4.
motorola began to lose interest in the G4, and apple looked to IBM to redevelop what would become the G5. it was brilliant, but then IBM also started to lose interest in the desktop pc market, being as they're now the main developer and supplier of all the chips in in the XBOX 360, the PS3 and even the Nintendo Revolution.
that's actually another example of the difference. all the major upcoming consoles will be using the RISC-based PowerPC line of processors. all major desktop computers will be using CISC.
so now, apple have made a major switch to using CISC-based x86 intel processors (AMD also make x86 processors). this means that all the PowerPC software won't run on intel, and intel software won't run on PPC, without an emulation layer, which is like having a french-spanish translator between the software and the processors translating the 'conversation' on the fly. this makes things slower, but workable (imagine having this conversation, if you are talking to someone in your own native languagel, the conversation is natural and flowing, with a translator to another language, the conversation is slower).
windows can't run on your G4 without emulation. look for Virtual PC, which runs windows on your G4 by emulating the CISC environment. it is very slow though.
Windows can run on the intel macs, and apple have let you do it.
mac OS X can't run on PC's, because apple are trying their best to stop you.