On his blog recently teknoid has posted a deeper look at using shells for the CakePHP framework that allow you to quickly make your application usable from the command line.

Why is that a good thing or what is it generally used for? Well, probably the #1 reason to write a shell is to allow certain aspects of your application to be executed by cron (behind the scenes), rather than through human intervention or web interface. [...] The other option is, of course, to be able to automate some mundane tasks... just take a look at the power of "bake".

He's broken things up into a few different categories - a general look at what the shells are and how they can be used, some of the more handy shell methods included by default, how to load in models to get access to your data and, finally, how to call a method from one of your classes.