Within the OS, the DA was in fact implemented as a special class of driver. It was installed in the driver queue, and given time periodically. A DA was permitted to have a user-interface as long as it was confined to one main window. A special appearance of window frame was reserved for the use of DAs so that the user could distinguish it from the windows of the hosting application.
Typical early DAs included the Calculator and Alarm Clock. Third-party DAs such as spelling checkers could be purchased. It was considered hard to write a DA, especially early on when there was little in the way of developer tools. However, since on the early Mac OS drivers did not have any special priveleges, writing a DA was, with practice, no more difficult than any other application.
With the advent of System 7, which included a standard co-operative multitasking feature, the need for DAs diminished greatly, and developers were encouraged to develop small applications instead. The system continued to run DAs (and still does up to Mac OS 9.x) for backward compatibility.