Python 3.3.2 and 2.7.5 first line code

Python 3.3.2 and 2.7.5 first line code

I am new to python. I have installed both version python 3.3.2 and 2.7.5
(Windows 7)
python 3.3 directory : c:\python33 python 2.7 directory : c:\python27
python33 was installed first.
What will be first line of code for both version of python? like as #!
/user/bin/python ??? more importantly why this line is important ?