Themes define the look and functionality of a WordPress site. There is a huge range of free WP themes that can be installed directly from the WordPress.org site. These themes can be customized to varying degrees. Themes offering more features and customization options can be bought at reasonable prices (£17 to £70) from sites such as:
Customizing these themes can be fiddly and it is often difficult to get just the look you want. For this reason I have a liking for the Artisteer theme design system which allows you to create simple themes without coding. Another theme design system worth looking at is Ultimatum.
The line between a very customizable theme and a theme design system is not distinct. Have a look at this article on theme builders on the Vandelay Design site.
Headway Themes has drag and drop capabilities and looks promising as a theme design system.