In the example below we have a variable c defined outside of any other function. In foo we also declare a c, increment it, and print it out. You can see that repeatedly calling foo() will yield the same result over and over again, because the c in foo is local in scope to the function.
In bar, however, the keyword global is added before c. Now the variable c references any variable c defined in the global scope (ie. our c = 1 instance defined before the functions). Calling bar repeatedly updates the global c instead of one scoped locally.
>>> c = 1
>>> def foo():
... c = 0
... c += 1
... print c
...
>>> def bar():
... global c
... c += 1
... print c
...
>>> foo()
1
>>> foo()
1
>>> foo()
1
>>> bar()
2
>>> bar()
3
