Hi
The above explanations are correct. Try this visual example as well to understand clear.
the CSS..
fieldset.login label
{
color:Blue;
}
fieldset label.login
{
color:Green;
}
the HTML...
<fieldset>
<label class="login">It is login class on the label</label>
</fieldset>
<fieldset class="login">
<label >It is login class in the FieldSet</label>
</fieldset>
This example is self explainable