Apple doesn't claim their computers never crash, just that they do so less frequently than computers running Windows. Which is true- I have never, in several years of use, seen OSX crash. Applications have crashed, but the system is resilient.
Apple FAQ wrote:
Is a Mac reliable?
When you buy a PC, you’re buying hardware from one company and an operating system and software from other companies. Not so with a Mac. Because Apple builds both the computer and the software that comes with it, they’re literally made for each other. This means that a Mac rarely freezes or crashes. Occasionally an application might quit, but it won’t affect the rest of your system. And Mac OS X resists most viruses, so you can do anything — without worrying about losing everything.
They said "rarely," not "never." But it's good enough that they can say "almost never." They took huge chunks of code from FreeBSD to make it, so it has a lot to offer in the way of stability. It even seems more stable than Ubuntu Linux, but that's probably just because I'm new to Linux and likely doing stuff wrong.
Oh, by the way: when an application crashes, you usually don't lose your work because it can still recover whatever you were doing. "Time Machine" does an automatic back-up at regular intervals to reduce the risk of losing something.
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WAR IS PEACE
FREEDOM IS SLAVERY
IGNORANCE IS STRENGTH