It used to be that there some some simply delineations within the computing space.

You bought Sun, you got SPARC computers running Solaris.
You bought Apple, you got PowerPC computers running Mac OS/Mac OS X.
You bought Compaq, you got a Intel-based PC running Windows.

Today the lines are blurring. Sun still sell SPARC kit, but they now also sell x86 kit based on Opteron processors. And these boxes don't just run Solaris, they also run Linux and Windows.

Compaq, now part of HP, still sell PCs, but they too will support Linux and Windows on them if you ask.

Apple, after years of PowerPC, are now moving to Intel-based PCs which will run Mac OS X, Windows, Linux, and Solaris.