It's an Operating System
Mac OS X is an operating system (OS) that manages just about everything on or connected to a Macintosh (Mac) computer such as files,  applications (programs), and devices such as monitors and printers. In essense, the OS defines how the computer will function and how it is presented to the user.  Be sure to read the What is a Mac guide for an overview of the Macintosh.

Mac OS X is the operating system for Apple's Macinosh computers and it is based on the Unix-based OPENSTEP operating system developed by NeXT Software which Apple acquired in the 1990's.  All new Macintosh computers run Mac OS X.  Mac OS X offers a powerful secure UNIX based core that is able to run a vast collection of software and presents to the user an easy to use graphical user interface (GUI) called Aqua.  Furthermore, Mac OS X allows users to take advantage of an open-source UNIX based opertating system along with the ease of use inherient to the Macintosh.

There are two key distinction of Mac OS X;  Mac OS X and Mac OS X Server.

Mac OS X
Mac OS X is designed for use on personal computers and is the version that comes pre-installed on all non-server based Macs.  For example, when you visit an Apple retail store all the Macs are running this version of Mac OS X as this is the version that most Mac users will use.  Note however that some Macs such as the Mac Pro give you the option to upgrade to Mac OS X Server at the time of purchase.

Mac OS X Server
Mac OS X Server on the other hand is Apple's server operating system and builds on the technology found in the consumer version.  It adds features such as a File and Print, Mail Services, Web Hosting, QuickTime Streaming, NetBoot & Network Install, as well as many other features.  This is the variant that enterprises and businesses are likely to use.

Mac OS X Releases

• Mac OS X 10.0.x "Cheetah" - March 24, 2001
• Mac OS X 10.1.x "Puma" - September 25, 2001
• Mac OS X 10.2.x "Jaguar" - August 24, 2002
• Mac OS X 10.3.x "Panther" - October 24, 2003
• Mac OS X 10.4.x "Tiger" - April 29, 2005
• Mac OS X 10.5.x "Leopard" - October 26, 2007
• Mac OS X 10.6.x "Snow Leopard" - August 28, 2009
• Mac OS X 10.7 "Lion" - Fall 2011
  

Updates to Mac OS X
Periodically, Apple releases "updates" to a release of Mac OS X.  These updates can include fixes, modifications, and security fixes.  When Snow Leopard was released it came out as version 10.6.0.  Soon after Apple released an update that took the version to 10.6.1.  You can install these updates via Software Update or with a stanalone installer.

Darwin
Darwin is the Unix-based core of the OS X operating system.  Darwin is an open-source stand-alone operating system.  Darwin is comprised of two major components: Mach 3.0 and BSD - Berkeley Software Distribution (FreeBSD).  Although Darwin is UNIX at the core, Apple created the beautiful and easy to use "Aqua" interface in Mac OS X to work with Darwin.  Note however that Darwin by itself wiil not run Mac OS X applications.

Mach
The Mach microkernel, originally developed at Carnegie Mellon University, is a core feature of the Darwin operating system.  It serves to manage the memory, tasks, and processes that run on a Mac.

Aqua
Mac users will not need to worry about Darwin, Mach, and FreeBSD as Apple has provided the "Aqua" graphical user interface (GUI) to use Mac OS X.  Most users will only use Aqua to do things in Mac OS X.  More advanced users will often use the Mac OS X Terminal to do more advanced work within Mac OS X via the command line.

Updates

• June 6, 2011 - content modification, added information about Lion, version 10.7
  
• March 24, 2010 - content modification, changed image to reflect Mac OS X Snow Leopard box
• February 26, 2009 - content modification
  
• July 13, 2008 - content modifications
  
• October 16, 2007 - Changed image to reflect Mac OS X Leopard box, content revision and corrections