Don't forget the dom is used in different smls, not just html.
In layman's terms the dom allows you to do crap with elements (<p>,<div>,etc.).
There could be more than just one <p> element on your page especially if your an exciting writer, so you usually want to give the <p> an identifier if you want to do this. Best way is to give it an id.
Now that's equivilant in html terms that the <p> element looks like this now.
<p id="myp" style="font-size:14px">
Now you want to make the font color green since its your favorite color. It'd look like this:
<p id="myp" style="font-size:14px; color:green">
There's other ways you can grab <p> and play with other elements, you are not confined to the id identifier. But that's the best way a noob like you would understand the concept.
Hope that helps some, jaspinal, cheers.