<!-- Standard button -->
<p><button type="button" class="btn btn-default">Default</button></p>
<!-- Provides extra visual weight and identifies the primary action in a set of buttons -->
<p><button type="button" class="btn btn-primary">Primary</button></p>
<!-- Indicates a successful or positive action -->
<p><button type="button" class="btn btn-success">Success</button></p>
<!-- Contextual button for informational alert messages -->
<p><button type="button" class="btn btn-info">Info</button></p>
<!-- Indicates caution should be taken with this action -->
<p><button type="button" class="btn btn-warning">Warning</button></p>
<!-- Indicates a dangerous or potentially negative action -->
<p><button type="button" class="btn btn-danger">Danger</button></p>
<!-- Deemphasize a button by making it look like a link while maintaining button behavior -->
<p><button type="button" class="btn btn-link">Link</button></p>

Os trechos em salmão são relativos aos nomes dos botões.