An excellent example demonstrating the difference between the various access modifiers can be found here and summarized below:
http://greenethumb.com/article/27/public-private-protected-internal-access-modifiers-in-as3/
public access modifier allows every Class access
// hey everyone, beer is on me!!!public function bar():Beer { ... }
internal access modifier allows Classes sharing this package access
//my good package-mates, have some beer on me!internal function bar():Beer { ... }
protected access modifier allows subclasses access
// I will only give beer to my children ( they are 21, i swear! )protected function bar():Beer { ... }
private access modifier allows no external access
// My beer is just that, MINE!private function bar():Beer { ... }
In Python, you use the underscore to define private methods. But they aren't really private, they're just mangled to hide them. A single underscore makes a variable or method private. A double underscore prevents a subclass from overwriting it and a double underscore before and after is reserved for Python iteslef.
Here is a better explanation: