CSS used for layout of your HTML requires DIV tags.
DIV tags are a grouping of a part of your HTML that
you want to have common traits. For example, say that
you wanted your links to be all the same color or the
same font-family. You would group all the HTML in a
DIV ID tag.
<Div id="example">
HTML CODE
HTML CODE
HTML CODE
</div>
When you define these tags in your CSS, the format is:
#example {property: value;}
Read more about this in the CSS basics page reached from
HERE.