The pythonpath tells python were to look for modules, for example you might have written a library that you want to use in several applications and stored it in the path /mylibs/python/
you would then have to add that path to the pythonpath for python to find it.
If you've downloaded a python module or library (I'm not really sure about the naming convention here) and you've just saved it in a random place on your computer, then you have to add it to your pythonpath.
However if you used easy_install or PIP then you dont have to worry.
To add something to the python-path in a *nix system you write:
export PYTHONPATH=$PYTHONPATH:/<path_to_modules>
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