On March 24, 2001, Apple released OS X 10.0 (Cheetah). Available for $129, OS X marked the beginning of the end for the classic Mac OS, and the rise of a new OS based on a UNIX underpinning.
In order to maintain compatibility with the large number of OS 9 applications in use, OS X was able to run a special 'Classic' compatibility mode that allowed OS 9 apps to run.
The initial release of OS X was not without its faults. The OS was slow, it had system requirements that many existing Macs were not be able to meet without upgrades, and it had a user interface that was dramatically different from the OS 9 interface that Mac users knew and loved.
But even with its faults, OS X 10.0 introduced Mac users to new features that would become second nature to end users: the Dock, a new way of organizing applications; Aqua, the new bold-color user interface, with 'lickable' buttons, a reference to brightly colored window buttons that Steve Jobs made during its introduction; Open GL; PDF; and, new for Mac users, protected memory. You could now run multiple applications without any application affecting the rest if it failed.
While OS X 10.0 had many problems, it created the foundation that all versions of OS X have since been built upon.