Cascading Style Sheets are a great way to control the look and feel of the site without having to repeat style code on each HTML element.

By defining styles for classes you can reuse style code over and over.  CSS classes are declared in a .css file or a <style></style> block by prefacing them with a period and giving them a name.

For example:

.my_css_class{

height:10px;

width:10px;

float:left;

}