What is the difference between sealed class and dynamic classes?

Sealed Classes: ActionScript 3.0 introduces the concept of sealed classes. A sealed class possesses only the fixed set of properties and methods that were defined at compile time; additional properties and methods cannot be added. This enables stricter compile-time checking, resulting in more robust programs. It also improves memory usage by not requiring an internal hash table for each object instance. Dynamic classes are also possible using the dynamic keyword. All classes in ActionScript 3.0 are sealed by default, but can be declared to be dynamic with the dynamic keyword.
Dynamic classes A dynamic class defines an object that can be altered at run time by adding or changing properties and methods. A class that is not dynamic, such as the String class, is a sealed class. We cannot add properties or methods to a sealed class at run time.