Well the big difference is that Wordpress is a content management system and Dreamweaver is a design software.
Wordpress has a dashboard which allows for easy updating and editing plus you can switch themes very easily and there's a plethora of widgets and plugins on the market (free and premium) to help better the user experience.
You have solid frameworks like Thesis and Genesis (those are premium, but there are free ones) which essentially are parent themes (the foundation). They are
SEO optimized, secure and when they update the framework, you automatically receive their updates via the parent theme.
You can then either purchase child themes, find free themes or create your own. If you want to create your own you have the choice of either creating one from scratch (which takes more time) or creating a child theme, which is based on a parent theme but you can modify it to your hearts desire and can end up with something that looks nothing like the parent.
Wordpress is so much less work than building a website from scratch and it's end-user friendly, so the client can update his website/blog on his own if need be.
Btw, Lynda.com rocks. I learned PHP and MySQL through them. You neethat at to make custom themes from scratch. Happy learning!