When Python parses a function, it notes when a variable assignment is made. When there is an assignment, it assumes by default that that variable is a local variable. To declare that the assignment refers to a global variable, you must use the global declaration.
When you access a variable in a function, its value is looked up using the LEGB scoping rules.
So, the first example
x = 1
def inc():
x += 5
inc()
produces an UnboundLocalError because Python determined x inside inc to be a local variable,
while accessing x works in your second example
def inc():
print x
because here, in accordance with the LEGB rule, Python looks for x in the local scope, does not find it, then looks for it in the extended scope, still does not find it, and finally looks for it in the global scope successfully.