In this code <div class=""> contents </div>
Note that bold one, it means the class of the div ur using. Between the double quotes (" "), you will put a name of a class created in the css u are using.
If u go to the css u are using in ur page, u will see something like
color : #ff0000;
thats ur class with the name header. So whe u write the code like this,
It means that that text header between the opening and closing div will have red color.
Do you understand what div class is now?