Bootstrap (also known as Twitter Bootstrap) is fairly light weight and makes responsive design and implementation very simple and straight forward. As I recently moved back into the front lines of development and found that I needed to understand the underpinnings of Bootstrap. While this tutorial was very good at helping me to understand the Bootstrap framework in building a wireframe, I became very aware of the other dependent technologies that Bootstrap uses. So I recommend that you get familiar with jQuery, Javascript, CSS3 and Modernizr, to name a few.
The people over a Tutsplus.com are doing a wonderful job in creating tutorials that are in-depth and effective. Take a look at this tutorial about Twitter Bootstrap, it just may brighten your day.