Where can I find an explanation for col- classes?

I'd really like to understand what the col- classes relate to and on what devices each one is used.

Looking at the page code there are a number of options:

col-xs-

col-sm-

col-md-

col-lg-

I understand that the numbers after this relate to the width within a row and must add up to 12 when used with columns. but what are the breakpoints for each to be activated?

Any chance of a page within the documentation to explain and highlight the standard CSS classes and their uses? It would really help when customising the template views.

Thanks

Lee