Macintosh OSX is the current version of the OS used on Apple Macintosh computers. For those who happen to pronounce it as "MAC OS X" they'll immediately be corrected by dedicated MAC users on the right pronunciation. While for some people the name can be a bit confusing, Apple's operating system is a user friendly and advanced OS that's aimed at making it easy for people to use their Apple computer. In general, it's both regular users and professionals that choose to use Apple's operating system mainly because of its stability.
The operating system's previous versions were based on the original Macintosh OS that was released in 1984. However, in the '90s, many people felt that Microsoft Windows has caught up on Macintosh OSX and they felt that Apple's OS started to lose its fans. In a response to that, Apple decided to fully revamp the Macintosh operating system and created a new one from scratch.
Even though a great part of the code that was used for building the new Macintosh OSX was written from scratch, it seems that a big part of it has been borrowed from the NextStep OS. This is basically a UNIX based OS which used to run on NeXT machines that are currently no longer produced. When Apple decided to acquire NeXT in '97, Steve Jobs was hired as interim Chief Executive Office and the Apple developers borrowed NEXTSTEP's UNIX based code and mixed it with the GUI of Mac OS nine.
The marriage of the two resulted in a high performance and extremely stable OS that had the intuitive interface of Macintosh OSX and the stability of UNIX. Soon after that in 2001, Macintosh OSX 10.0 was released.
Since its initial release, several major updates have been released by Apple in an attempt to improve their operating system even more. The updates have always brought something new and improved on functionalities that users have been asking for through Apple's dedicated forums. Apple's current OS dubbed "OSX Mavericks" is available as a free download on its official website.