Recurse Directories and process files in directory

[KraftDiner]

Hi I need help writing a python script that traverses (recursivly) a
directory and its sub directories and processes all files in the
directory.  So at each directory if there are files in it I must build
a list of those files and process them by exectuing a system command
(exec?)

Can some one tell me what methods to use to:
a) Walk the directory tree
b) execute a system command with parameters.

TIA.