The base of OSX is BSD, an operating system designed at Berkley University (Hence Berkley Software Distribution). On top of BSD is a set of technologies created by Apple and named ‘Darwin’, due to the evolution of the OS that these technologies enabled. BSD is very similar to UNIX, the back-end, geek-only operating system that also forms the basis for the mainstream (at least by comparison) Linux.
Because of the way that BSD and Linux is written, it is inherently more secure and inherently more stable than Windows. Up until OSX, the problem had been that despite these advantages, it was horrendous complex to use. When Apple came along and paired BSD with Darwin and its own user interface, it struck gold.
Today, the Apple marketing pitch is very much that OSX is more stable, faster, more feature-packed and easier to use than Windows XP. In the vast majority of cases, this is absolutely true, in our experience – you only need to look at the number of security flaws and bugs documented for Windows XP over the past year and compare them to OSX. Where Apple falls down is in the speed of its hardware compared to Intel and AMD, but that is a different story.