In order to eliminate the technical limitations of Mac OS 9.x, itself based on the original 1984 Mac OS, Apple needed to create an entirely new subsystem. Mac OS X introduced that new subsystem, based on a combination of BSD UNIX and NeXT/OPENSTEP. In the process of doing this, most of the GUI was also given a fresh coat of paint (or glass).
As you can see, the UI has now been given a glass-like appearance. The UI also includes many other forms of "eye candy", such as animations, background gradients (like the pinstripe you can see here in the background of the dock and menu bar) and transparency. This "eye candy" serves no technical purpose at all but it pleases the new computer user so that is why it exists.
One of the new features in OS X is the Dock, which provides an easy way to launch programs and to access currently open applications as well.
One thing new in OS X is the ability to minimize windows. Mac OS 9 did not have this functionality due to the limitations of the 1984 Mac.
Minimized windows appear as a preview of the window in thumbnail form on the right side of the dock.
The Finder features the same Icon and Detailed List views as before, and thankfully is not a web browser, a major difference from certain other operating systems, and for the better.
One new thing with the Finder, however, is the Column view, which comes from NeXTSTEP. The column view starts at the Computer level with the drives, and has a new column for each level of the directory tree you browse. The last column displays file information, as well as a preview if that is applicable.
Unlike previous versions of Mac OS, the Close, Minimize and Maximize buttons are all on the left side of each window. The X, - and + symbols are hidden until the user mouses over the buttons, which helps reduce desktop clutter.