Personal Computer. Despite its definition, PC's were originally computers with Intel processors. Most people think that PPC is synonymous with Windows, however, those people are idiot when it comes to computers. PC includes Windows, any variant of Linux (that is a lot of operating systems), BSD, Solaris, and all Unix-based operating systems. Macs are Unix based, so when people compare Macs to PCs, they are comparing the same thing. This is not because Macs are "Personal" (Windows is more common in workplaces now), but Macs match every other of today's definition of PC.
Mac users need to STFU. Macs are PCs, and Windows always have (can't say always will be) been better. The best PCs are not Windows, however.