Control panel

A control panel (CDEV) is an item under classic Mac OS, but not Mac OS X or macOS, which lets Macintosh users adjust system preferences and settings. Control panels can contain code that is loaded at startup, like an extension.

Early Mac OS
Individual control panels were preceded by the Control Panel desk accessory, which dates back to the first System software release in 1984. From System 4 to 6.x, this desk accessory's features could be extended with CDEV files, which were loaded from the System Folder upon startup.

System 7 to Mac OS 9.2.2
From System 7 to Mac OS 9.2.2, control panels are stored in their own Control Panels folder within the System Folder and can be individually activated or deactivated through Extensions Manager. Unlike System 6.x and earlier, the control panels can be launched simultaneously into their own individual windows.

Mac OS 7.6 introduced control panels that were mini-applications in their own right. These are the so-called "APPC" control panels.

Replacement
With the introduction of Mac OS X (now macOS), control panels were replaced by System Preferences. Control panels from Mac OS 9.x can, however, still be used (with some limitations) under the Classic environment until Mac OS X 10.4.11.