How does twitter bootstrap work internally

I have basic understanding of CSS,HTML and better of scripting languages Javascript,PHP... I am trying to dive into Bootstrap framework. Documentation great, any problems without creating pages, everything works pretty well.

But I really hate doing something without understanding how does this work under the hood internally.

I understand that CSS3 is used with browser specific features, but I cannot leave this as it is, I want to get better understanding of bootstrap.
And how can I implement the same without using bootstrap.css.

I know the best way is just to read bootstrap.css but maybe someone has link to article or any other resource explaining how all this stuff works and how this is achieved and I can implement without bootstrap.

Thanks.