From The Free On-line Dictionary of Computing: (http://www.foldoc.org/, Editor Denis Howe)
(PPC) A RISCmicroprocessor designed to meet a standard which was jointly designed by Motorola, IBM, and Apple Computer (the PowerPC Alliance). The PowerPC standard specifies a common instruction set architecture (ISA), allowing anyone to design and fabricate PowerPC processors, which will run the same code. The PowerPC architecture is based on the IBM POWER architecture, used in IBM's RS/6000workstations. Currently IBM and Motorola are working on PowerPC chips.
The PowerPC standard specifies both 32-bit and 64-bit data paths. Early implementations were 32-bit (e.g. PowerPC 601); later higher-performance implementations were 64-bit (e.g. PowerPC 620). A PowerPC has 32 integer registers (32- or 64 bit) and 32 floating-point (IEEE standard 64 bit) floating-point registers.
The POWER CPU chip and PowerPC have a (large) common core, but both have instructions that the other doesn't. The PowerPC offers the following features that POWER does not:
The real time clock (upper and lower) was replaced with the time base registers (upper and lower), which don't count in sec/ns (the decrementer also changed).
64-bit instruction operands, registers, etc. (in 64 bit processors).