In the Mac OS, a folder in the System Folder. Anything placed in this folder will be automatically launched on startup. You can place files, folders, programs, or aliases in the folder. Very convenient if you have a core set of applications that you use every day, or if you like to have certain programs or utilities always running in the background (screen savers, Serial Port Monitor, etc.)
You can switch off items in the Startup Items folder through the Extensions Manager interface. Items that you have "switched off" get moved to a folder called "Startup Items (Disabled)".