On JoomlaBlogger.net there's a recent post that wants to help you understand the Joomla templating system in "five easy steps".

There are very few tutorials on how to build templates. There are two books and a handful of other resources scattered around the web. So, we set out to build the very best template class we could. Here are first five sessions of the class: Joomla templates are not rocket science. You can learn how to build and modify them. We can show you how. Interested?

They break it up into the basic parts:

• Template setup and templateDetails.xml
• Index.php
• CSS/Cascading Style Sheets
• Images
• Security