Applies to Bootstrap 3 only.
Ignoring the letters (xs, sm, md, lg) for now, I'll start with just the numbers...
col-*-6 spans 6 of 12 columns (half the width), col-*-12 spans 12 of 12 columns (the entire width), etcSo, if you want two equal columns to span a div, write
<div class="col-xs-6">Column 1</div>
<div class="col-xs-6">Column 2</div>
Or, if you want three unequal columns to span that same width, you could write:
<div class="col-xs-2">Column 1</div>
<div class="col-xs-6">Column 2</div>
<div class="col-xs-4">Column 3</div>
You'll notice the # of columns always add up to 12. It can be less than twelve, but beware if more than 12, as your offending divs will bump down to the next row (not .row, which is another story altogether).
You can also nest columns within columns, (best with a .row wrapper around them) such as:
<div class="col-xs-6">
<div class="row">
<div class="col-xs-4">Column 1-a</div>
<div class="col-xs-8">Column 1-b</div>
</div>
</div>
<div class="col-xs-6">
<div class="row">
<div class="col-xs-2">Column 2-a</div>
<div class="col-xs-10">Column 2-b</div>
</div>
</div>
Each set of nested divs also span up to 12 columns of their parent div. NOTE: Since each .col class has 15px padding on either side, you should usually wrap nested columns in a .row, which has -15px margins. This avoids duplicating the padding and keeps the content lined up between nested and non-nested col classes.
-- You didn't specifically ask about the xs, sm, md, lg usage, but they go hand-in-hand so I can't help but touch on it...
In short, they are used to define at which screen size that class should apply:
Read the "Grid Options" chapter from the official Bootstrap documentation for more details.
You should usually classify a div using multiple column classes so it behaves differently depending on the screen size (this is the heart of what makes bootstrap responsive). eg: a div with classes col-xs-6 and col-sm-4 will span half the screen on the mobile phone (xs) and 1/3 of the screen on tablets(sm).
<div class="col-xs-6 col-sm-4">Column 1</div> <!-- 1/2 width on mobile, 1/3 screen on tablet) -->
<div class="col-xs-6 col-sm-8">Column 2</div> <!-- 1/2 width on mobile, 2/3 width on tablet -->
NOTE: as per comment below, grid classes for a given screen size apply to that screen size and larger unless another declaration overrides it (i.e. col-xs-6 col-md-4 spans 6 columns on xs and sm, and 4 columns on md and lg, even though sm and lg were never explicitly declared)
NOTE: if you don't define xs, it will default to col-xs-12 (i.e. col-sm-6 is half the width on sm, md and lg screens, but full-width on xs screens).
NOTE: it's actually totally fine if your .row includes more than 12 cols, as long as you are aware of how they will react. --This is a contentious issue, and not everyone agrees.