Extensions Managing Them On Mac OS 9 And Earlier

A relatively esoteric skill, it was still prized amongst Apple Macintosh users. Starting with System 7, Apple added an easier method for extending the capability of the operating system using a plug-in architecture. Called “extensions”, these miniature programs added drivers and otherwise modified the operating system during the boot process. However, conflicts could occur as extensions overwrote one another, and many Mac users learned how to manage the ever-growing list of extensions by hand, or to use commercial solutions like Conflict Catcher.

With the adoption of Mac OS X, the need to manage extensions dwindled, and as more developers wrote their applications for Mac OS X, the need to invoke the Classic environment shrank as well. Finally, with the switch to intel processors, the Classic environment was also no longer a part of the Mac OS system, and this skill of tweaking extensions became obsolete.

See this document for more: http://docs.info.apple.com/article.html?artnum=30929(approve sites)

As a side note, this skill is related more to managing plug-ins in applications like Adobe Photoshop and QuarkXPress?, and less like managing extensions to other operating systems or to Unix applications like Apache.