The diff is pretty awful but the change is basically going from this:

<div id="leftcolumn">
    ...
</div>
<div id="rightcolumn">      
    ... 
</div>

(notice also that IDs were used which is not ideal because it's sometimes reused many times in the same page)

To this:

<div class="row">
    <div class="col-md-6">
        ...
    </div>
    <div class="col-md-6">
        ...
    </div>
</div>

To learn more about Bootstrap classes read the documentation here: http://getbootstrap.com/css/#grid-example-basic

Another cool thing is that we can now have more than 2 columns, create more advanced layouts (nested columns) and also choose the width at which columns must collapse.

Here are some screenshots when the window is resized to something small (not the same Piwik so don't look in details it doesn't match it's normal).

Current behavior (not responsive)

New behavior (responsive)

this has been an issue for a long time. Nice change!

• a report was created since the PR in https://github.com/piwik/piwik/blob/replace-columns-with-bootstrap/plugins/DevicesDetection/templates/software.twig#L6-6 and it's using leftcolumn

software.twig updated in the PR

nice, this is an improvement that will please many users