Cast some shadow below buttons.
Example
<button type="button" class="btn btn-default btn-raised">Raised default button</button>
<button type="button" class="btn btn-primary btn-raised">Raised primary button</button>
<button type="button" class="btn btn-accent btn-raised">Raised accent button</button>
<button type="button" class="btn btn-success btn-raised">Raised success button</button>
<button type="button" class="btn btn-info btn-raised">Raised info button</button>
<button type="button" class="btn btn-warning btn-raised">Raised warning button</button>
<button type="button" class="btn btn-danger btn-raised">Raised danger button</button>