Demo:

id=unique element,  defined by #
class=repeating element, defined by . (period)

html:

<div id=”top-images”>
<p class=”caption”>
Image Caption
</p>
</div>

css:

#top-images{
width: 600px;
text align: left;
position: relative
}

p.caption {
font-size: 10px;
text-decoration: underline
}