MacDevCenter.com: An Introduction to Tiger Terminal, Part 5:

In previous parts of this Tiger Terminal series (Part 1, Part 2, Part 3, & Part 4), I introduced the OS 10.4 version of the terminal.app, shell basics, CLI network utilities, and shell scripting. In Part 5, I'll show you how Tiger Mac OS X runs regularly scheduled commands and scripts to execute recurring jobs, like system maintenance and backups by comparing the "old" way, using the Unix tool called cron (for chronological), with the new Tiger method of using the launchd daemon.