Apple has recently renamed “Mac OS X” to “macOS”. Mac OS X / macOS is a UNIX operating system based off of the BSD UNIX operating system and initially developed by NeXT Computer as NeXTStep. When Steve Jobs returned to Apple from his time at NeXT, he brought the operating system with him as the replacement for Apple’s System 9 operating system to use in Apple products. Mac OS X debuted in 2002. The operating system consists of a kernel called XNU which is based on a fusion of the Mach micrkernel and the FreeBSD operating system. The kernel, combined with the FreeBSD libraries and runtime applications a is called “Darwin”. On top of Darwin, Apple has added their own GUI system called Quartz and a slew of application frameworks and services that, combined, are called Mac OS X / macOS. The operating system is only licensed to be used on Apple-branded hardware, so you only generally see it on Apple computers. Apple also ships stripped-down versions of macOS that are based on Darwin, but with different application frameworks. These are found on other Apple products. iOS (phones and tablets), TvOS (the Apple TV streaming box), and watchOS (the Apple watch).
In Closing If I just had a MacBook as my main machine, I think I would use it for the majority of my day-to-day apps and work, though I would certainly use it for some gaming. I think that for me, the MacBook provides a great balance between the power (MacBook and the power of the OSX environment) and the portability (the power of the OSX environment and the power of the MacBook). I was not sure if I would really enjoy using a MacBook for gaming and other desktop tasks, but after a week with MacBook Pros, I am quite happy. So, if you have an interesting idea about what to use to replace your current Mac, and you believe your old machine could benefit from a new OSX based machine, then by all means, do some research and see what you come up with.