Right in the page you referenced was a link, "The Aqua Interface". Following that link yields an explanation.
Aqua is the overall appearance and behavior of Mac OS X. Aqua defines the standard appearance of specific user interface components such as windows, menus, and controls and is also characterized by the anti-aliased appearance of text and graphics, shadowing, transparency, and careful use of color. Aqua delivers standardized consistent behaviors and promotes clear communication of status through animated notifications, visual effects, and more. Designing for Aqua compliance will ensure you provide the best possible user experience for your customers.
Aqua is available to Cocoa, Carbon, and Java software. For Cocoa and Carbon application development, Interface Builder is the best way to begin building an Aqua-compliant graphical user interface. If you are porting an existing Mac OS 9 application to Mac OS X, see the Carbon Porting Guide in Carbon Porting Documentation. Java developers can use the Swing toolkit, which includes an Aqua look and feel in Mac OS X.
Read this part to learn about what Aqua provides and how best to take full advantage of it to ensure your application feels completely “at home” in Mac OS X.
In addition to the resources at the site, Wikipedia has an article on the Aqua Interface as well.
Aqua is the GUI and primary visual theme of Apple Inc.'s Mac OS X operating system. It is based around the theme of water, as its name suggests, with droplet-like elements and liberal use of translucency and reflection effects. Steve Jobs noted Aqua's glossy aesthetic: "One of the design goals was when you saw it you wanted to lick it."
The Aqua theme and user interface was first introduced at the January 2000 Macworld Conference & Expo in San Francisco. Aqua's first appearance in a commercial product was in the July 2000 release of iMovie 2.
Aqua design elements make up the uniform appearance of most Mac OS X applications. Its goal is to "incorporate color, depth, translucence, and complex textures into a visually appealing interface" in Mac OS X applications. Although Aqua is the entire user interface, two notable features of Aqua are gel-like buttons (such as the ones colored red, yellow, and green that control the windows), and a Dock, which facilitates the launching of and navigation between applications.
Perhaps this question would be better in a computer-related forum.