SpringBoard is the application that manages the home screen on iOS devices. With SpringBoard, icons on the home screen can be organized, the background wallpaper changed, and applications are launched. Some of the iPhone's settings at startup are also set by the SpringBoard.
Essentially SpringBoard is like the mobile version of a desktop. Mac OS X features the Finder while Windows computers have the Explorer. In iOS 4.3.3 firmware, SpringBoard looks for apps to display on the home screen in the /var/mobile/Applications and /Applications directories on the iPhone.