Wikipaedia is your friend. Emphasis is mine.


Mac OS X (pronounced /ˈmęk ˌoʊ ˌɛs ˈtɛn/)[7] is a series of Unix-based operating systems and graphical user interfaces developed, marketed, and sold by Apple Inc. Since 2002, Mac OS X has been included with all new Macintosh computer systems. It is the successor to Mac OS 9, released in 1999, the final release of the "classic" Mac OS, which had been Apple's primary operating system since 1984.
Mac OS X, whose X is the Roman numeral for 10 and is a prominent part of its brand identity, is a Unix-based graphical operating system,[8] built on technologies developed at NeXT between the second half of the 1980s and Apple's purchase of the company in late 1996. From its sixth release, Mac OS X v10.5 "Leopard" and onward, every release of Mac OS X gained UNIX 03 certification while running on Intel processors.[3][4] Mac OS X - Wikipedia, the free encyclopedia

I am curently running Snow Leopard and will switch to Lion after another 6 months or so. A pilot once told me"Never own the -A Model". Wise words.