new font system in .Net?

Can someone please explain to me what's happenned to the fonts when you're developing a web application? I''ve got my .css file and it reads as per below. *** does 1em and 2em and 3em mean? I can understand that you can manually change things to 10pt etc, but would like to know what em means. Also, is there a way to make it default to pt?

cheers.

Code:

---

BODY
{
        font-weight: normal;
        font-size: 8pt;
        word-spacing: normal;
        text-transform: none;
        font-family: Verdana, Helvetica, sans-serif;
        letter-spacing: normal;
        background-color: white;
}

H1, H2, H3, H4, H5, TH, THEAD, TFOOT
{
    COLOR: #003366;
}
H1        {       
        font-family: Verdana, Arial, Helvetica, sans-serif;
        font-size: 2em;
        font-weight:        700;
        font-style:        normal;
        text-decoration:        none;
        word-spacing:        normal;
        letter-spacing:        normal;
        text-transform:        none;
        }       
               
H2
{
        font-weight: bold;
        font-size: 10pt;
        word-spacing: normal;
        text-transform: none;
        font-style: normal;
        font-family: Verdana, Arial, Helvetica, sans-serif, 'Trebuchet MS';
        letter-spacing: normal;
        text-decoration: none;
}

---

I have almost never even opened a web project and even less aStyle sheet, so forgive my ignorance. :)

What it seems though is that em and ex is a relative units designator opposed to an absolute units designator like for example cm, mm or pt. I think this is relative to the deafult size of the object being measured. Read my first sentance though, this is a mere guess. :)