Okay, I have no way of comparing it to a guy's orgasm because I have no clue what that feels like. For girls, well for myself (I shouldn't speak for all girls, I guess), this is how it is:
It starts like a slow burn from the inside out that starts in the obvious location and spreads out...down my legs to my toes, up through the pit of my stomach, like a trail of hot wax up my spine to my neck, skin all tingly all over. Then, it is a series of...well, you know that feeling when you are driving fast and go down a steep hill and then rapidly up another one? That feeling.

A series of those, and then finally like someone has dumped a huge bucket of warm, warm water all over you, followed by an absolutely electrifying feeling all over, inside and out.
Hahahaha...that sounds so silly. I don't think it's really possible to describe it. You just have to feel it.