Joomla And WordPress: A Matter Of Mental Models - Smashing Magazine
http://www.smashingmagazine.com/2010/05/03/joomla-and-wordpress-a-matter-of-mental-models/

Really nice article on the models behind Wordpress and Joomla.

WordPress’ extensions model is based on the execution of a set of functions attached to the system flow by mean of “hooks.”

Compares and contrasts templates and other aspects of these two systems. Might be useful if I ever need to get up to speed on Joomla! quickly.