Start-up items are procedures run during the last phase of booting to prepare a Mac OS X system for normal operation. They consist of programs (including shell scripts) that perform tasks such as clearing away temporary files and starting system daemons. The system start-up items (that is, those provided by Apple) are located in /System/Library/StartupItems. Do not modify the items in this directory. However, you can define your own start-up items; these custom start-up items are stored in /Library/StartupItems.
The SystemStarter programs, which is the last thing run by the rc script, coordinates the execution of all start-up items. A start-up item is a folder containing at least two files:
A program (typically a shell script) that takes the same name as the folder
A configuration property list
The property list file for each start-up item is named StartupParameters.plist. The property list specifies the name of the star-tup item and the dependencies for the star-tup items at multiple levels of granularity. These values indicate the services that the start-up item provides, which services must be run before the start-up item, and which services the start-up item uses (if available). SystemStarter reads and processes the dependency information in all property lists and determines the order in which to run the scripts or programs in the folders.