What does mod_mp3 do?

Hi,

I've been fooling around a bit with mod_mp3 and am now confused. So this
may be a stupid question, be warned! 

What does mod_mp3 do exactly when somebody clicks on a url like:
http://servername/?op=play&song=<md5(filename)>
Is it any different from clicking on
http://servername/filename.mp3 
when apache knows about the mime-type of a file with suffix .mp3 (so
without any mod_mp3 interference)?

It seems that in both situations apache sends something with
content-type: audio/mpeg...

Thanx,

Tim