They were run initially at start-up time, and operated by a variety of mechanisms, including trap patching and other code modifying techniques.
Initially an Apple developer hack, extensions became the standard way to provide a modular operating system.
Large amounts of important system services such as the TCP/IP network stacks (MacTCP and Open Transport) and USB and FireWire support were optional components implemented as extensions.
Since taking advantage of this mechanism was an unsupported hack, and only 32 INITs could be loaded in this manner, Apple responded by providing a more managed solution.
INITs evolved into system extensions, gaining additional ad hoc protocols along the way, such as supplying an icon to be displayed at boot time (the origin of this was 'ShowINIT').
A substantial number of services and drivers in Mac OS—both official and third party—were provided as extensions, allowing for the OS to be trimmed down by disabling them.
The simplest way to clean-boot the operating system was to hold the shift key: loading of extensions would be bypassed.
MultiFinder and System 7 and later supported faceless background applications similar to UNIX daemons or Windows Services, though using cooperative multitasking.
The Control Strip in Mac OS 8 and 9 was an example of a faceless background application that displayed a global floating window to provide user interaction.
However, the user was not aware at any time that the Control Strip was a running process; it was simply presented as an extra interface feature.
The BootX Linux bootloader was implemented as a scri simply because such files were loaded very early on in the boot process, before all other extensions.