The second file (style.css) contains all of the specific stylings for the page. The file is separated into sections using:
/* Structure */
/* Header */
/* Sidebar */
/* Footer */
If you would like to edit a specific section of the site, simply find the appropriate label in the CSS file, and then scroll down until you find the appropriate style that needs to be edited.
Third one is responsive.css which contains all the code which is required to provide responsiveness. More information about Twitter Bootstrap framework you can find here
In /css folder you will also find ie7.css, ie8.css and ie9.css which are supposed to work only in IE7, IE8 and IE9 browsers.
All of them is basically the same as with only difference that it’s supposed for IE7, IE8 and IE9.
Also you will see that in /css/color-schemes/ is specific stylesheets for each color scheme