CSS >> Bootstrap >> Row
.row defines a row of divs, like the name implies. Each one indicates a new line of divs, no matter if the above line is full or not
<div class="container">
<div class="row">
<!-- These divs are inline and do NOT fill up the full 12 columns -->
<div class="span4">...</div>
<div class="span4">...</div>
</div>
<!-- This is a automatically a new line of divs -->
<div class="row">
<!-- This div will appear below the previous row, even though it
would fit next to the other divs -->
<div class="span4"></div>
</div>
<!-- These will appear in their own row, but may act
unexpectedly in certain situations -->
<div class="span4"></div>
<div class="span4"></div>
</div>