at the moment, and for the last 10 years, macs have used a processor designed by IBM called the PowerPC, derived from their Power architecture of server chips. the G5 refers to the PowerPC 970, which is the 5th Generation of PowerPC chips.
PC's have always used x86 processors. this family started with the intel 8086 in the late 80's/early 90's. this was replaced by the 286, which in turned was replaced by the 386 processor, the 486 processor, and the 586 processor, which was actually called simply Pentium, to avoid copyright issues (you can't copyright a number).
The pentium became Pentium II and AMD around this point started making their rival, the K6, which was replaced by the K7 Athlon, when intel released Pentium III.
so x86 refers to the chip architecture that was intel's way of doing things. AMD are a rival company that makes compatible chips on the x86 architecture. IBM's PowerPC is drastically different, and they are moving out of the pc market into consoles (the PPC is basis for the chip that all the next gen consoles will be using, Xbox360, PS3 and nintendo revolution). apple have decided to switch to x86 intel chips, starting this year. the Mac Mini is rumoured to be one of the first to cross over.