Best to show with an example:
<p class="classone">some text</p>
<p class="classtwo">some text</p>
<p class="classone classtwo">some text</p>
<style type="text/css">
.classone{
color:#FF0000;
}
.classtwo{
color:#00FF00;
}
.classone.classtwo{
color:#0000FF;
}
</style>
Preview:
some text
some text
some text