working with files with the same extenstion

[Ulrich Pfisterer]

Can anyone please tell me how I would go about doing the following.
I would like to write a script that would take in a command line 
parameter "*.txt". The script would then loop through every file with a 
"txt" extension and something with it.

I am brand new to python (I started using it 2 days ago) so I dont know 
if there is a module that does already this, or whether I have to do 
all the wildcard matching etc myself.

Any help would be much appreciated