The WP For Developers Series was posted on the ThemeForest blog a while back, and it’s a pretty solid set of tutorials by Drew Douglas. It covers everything from installing a local installation of WordPress (incredibly useful if you plan on doing much developing/customizing of themes) to creating all of the necessary files for a functional theme to building a simple plugin.
If you’re looking to learn how to develop your own themes, these videos are perfect. If you just want to be a little more comfortable next time you look “under the hood” of your own blog, this will make your tweaking much, much easier. Think of it as a guided tour of the guts of your blog, so that next time you need to change something, you’ll know what you’re looking at!
The initial post (linked above) contains links to the first eight videos, but don’t be fooled! There are 18 videos in total, and they aren’t hard to find.