nope, they are different hardware from PC's, although that gap is now starting to close since they started using x86 varient based CPU's last year instead of RISC CPU's.
They run MacOS, the current series being MacOS X or better know nowadays as simply OSX. I think the latest release is Tiger (aka MacOS 10.4), but Leopard is out soon to replace it (aka MacOS 10.5).
Its a lovely OS is OSX, although MacOS 9 and earlier were good, OSX has made inroads on the dominence of Microsoft Windows, and is more secure, more reliable and more efficient, although obviously it also has drawbacks, the main being lack of software availablility, which has gotten better over recent years.