I Adore Twitter’s Bootstrap.
I find that when developing with it, that I usually need to apply some custom styles at the different screen/device sizes.
Bootstrap is a mobile-first framework. Included below are the correct order for the Media Queries, and below those are the non-mobile first breakpoints.
Min-Width: Refers to everything greater than or equal to the amount given.
Max-Width: Refers to everything less than or equal to the amount given.
If you use SCSS, SASS or LESS, you can use the default variables for breakpoints.