Well, before Mac and PC used to be very different.
They used to be different architectures.
PC running on x86, and Mac running on PowerPC (PPC).
PC running AMD/Intel and having MMX. While Mac running Motoral or IBM with Altivec.
But now, Mac and PC are basically the same thing, both are running on the x86 architecture nowdays. So basically Mac have left its Mac roots and become a x86 PC.
Mac are said to be overpriced, and they might very well be, I read something a while ago, that it was actually as cheap as any other PC. But they could be very well be overpriced, Mac is a bit of niche.
With a Mac, you get a cool looking computer case in brushed metal. You can also run Mac OS X which seems to be a pretty decent operating system.
Mac seems to have some smart features and been well thought-out. Or atleast thats what their marketing believe you think.

See:
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http://www.apple.com/macpro/*
http://www.apple.com/macosx/