In simple words, Aqua is the interface the end user uses and it's what gives Mac OS X its look and feel.  Aqua is all about user experience because it determines how things are presented to the end user, it makes it easy to manage one's work, and it helps one use various applications.

Aqua gives Mac OS X a sleek smooth look, vibrant colors, translucent buttons, clean high-resolution icons, menus, and drop shadows.  Aqua also gives Mac OS X stunning visual effects such as transparency, reflections, and animations.

Those new to the Mac will immediately notice the brushed metal look and the bright droplet like buttons on the upper left hand corner of each window.  An alternative Aqua theme is called Graphite, a grayscale look and feel.

Aqua's interface controls are more sophisticated than any other operating system thanks to the Quartz Compositor, the advanced graphics processing technology. Quartz Compositor was extended in Mac OS X 10.2 Jaguar to what was termed Quartz Extreme.  In OS X 10.4 Tiger, Apple released Quartz Extreme 2D, a more advanced graphics engine.  Both Quartz Extreme 2D and Quartz Extreme use OpenGL in conjunction with graphics cards to render windows as textures through the Aqua interface.  This allows graphics to be processed faster and takes processing load off of the CPU (processor).  The CPU can then concentrate on processing tasks associated with applications and the operating system.  Overall, it's a much better way to offload work so your Mac will run faster and more efficiently.

Mac OS X Leopard has brought improvements to Aqua including  Coverflow and more refined/consistent window themes.  Additionally, running applications are now indicated on the Dock by either a glowing blue ball or a white dot.  Note that the Aqua theme has also been included in Microsoft Windows versions of Apple applications such as iTunes, Quicktime, and Safari.

Thanks to Aqua the OS X environment will be relatively familiar to new Mac users because it provides users with features such as a desktop, windows, folders, menus, and keyboard shortcuts.  Navigating through Mac OS X is easy and straight forward. Aqua includes the following "major" components that all inter operate with each other to allow a user to work on a Mac:

Other Aqua features tied to the end user experience include:

• Automator
• Dashboard
• Exposé
• Fast User Switching
• Inkwell
• Speech
• Spotlight
• System Preferences
• VoiceOver

All of these features and technologies have a key role in how your interact and use a Mac.  Each release of Mac OS X has not only ushered in advancements to existing Aqua components and features but news ones as well. Mac OS X Tiger (10.4) for example added features such as Dashboard and a new search feature called Spotlight.  Mac OS X Panther (10.3) added several features one of them was Exposé.

Aqua also implements consistency.  All keyboard shortcuts are the same across applications thanks to the "command" key shown to the left.  Mac keyboard shortcuts use the command key in conjuction with other keys to invoke an action.  For example when the Apple key and the P key are pressed at the same time, it tells OS X to perform a Print.  Just try it.  Open a document, a web page, and an email.  Perform the action in each application to see that print is always invoked with that combination.

More advanced users will often bypass Aqua and use the Mac OS X Terminal to do more advanced work within Mac OS X via the command line.

Updates

• February 26, 2009 - content modification
  
• July 13, 2008 - content modification, updated image with new command key