Ok, it's stupid question time...
I just bought a "new to me" '01 S40.... The antenna has been snapped off.

I have looked around online to order a new antenna, an apparently there is more to this than I realize. How do I know which antenna I need?

 

Don't believe the advertising about "universal" antenna's. They don't work without much fitting and are usually loud. Buy genuine Volvo and you will get high quality and low noise. Expensive but worth it. I bought mine from the local dealer and it works like showroom new. Here is another source:

(OES = Original Equipment Supplier)
VOLVO S40 ANTENNA COMPONENTS 2000-2012 at Swedish Auto Parts

EDIT: For the record I am referring to the whole unit, not the mast replacement.

 

In my 850's and 960's I always got the cheap $15 one they offer and have never had a problem.