Theme Frameworks are very flexible themes, they allow you to create “child themes” based on the functionality of the theme framework. So you don’t have to spend so much time coding your theme. Some Frameworks are so flexible, that all you need to change is the CSS, and you’re off to the races. The trick is finding one with features you desire.
(Warning to my Students: don’t get sucked in to the lure of never looking at the code, the more you know the more flexible & tailored you can make your theme. By all means stand on the shoulders of giants, but be sure you know enough to whisper in their ear, and get them headed in the direction you would like to go)
Some popular Theme Frameworks:
How To Create a WordPress Theme: The Ultimate WordPress Theme Tutorial
Creating a WordPress theme with WP Framework:
How to make a child theme for WordPress: A pictorial introduction for beginners