We use ID and CLASS selectors to get the job done for us...
We use them like this :
<p> This content goes into the box on the right hand side of the page </p>
border: 2px solid black;
To style div id's that you make, you <span style= “background-color: #FFFF00″>use the "#" sign</span> in the CSS file...
To style div classes that you make, you <span style= “background-color: #FFFF00″>use a "." (period)</span> in the CSS file
IDs are used when you are making it like a whole different tag like in the example above, but CLASSES are used when you want to style things inside another tag, like
<p> This <div class="MYFIRSTDIVCLASS"> text </div> is styled by div class...</p>
So classes are used when you have the specific div class inside another HTML tag (Excluding HTML, Body and Head tags)
Pre-defined elements like (h2, p, h1, br, em, i, b etc...) are edited using the div class function because they are repeated a lot of times in the document, and therefore it has to be a class div...
Div IDs are used when you are going to use the styles element only once in the HTML document...