Decorator is a structural pattern that consists of dynamically augmenting the behavior of an existing object. It's different from classical inheritance, because the behavior is not added to all the objects of the same class but only to the instances that are explicitly decorated.
Implementation-wise, it is very similar to the Proxy pattern, but instead of enhancing or modifying the behavior of the existing interface of an object, it augments it with new functionalities, as described in the following figure:
In the previous figure, the Decorator object is extending the Component object by adding the methodC() operation. The existing methods are usually delegated to the decorated object, without further processing. Of course, if necessary we can easily combine the Proxy pattern, so that also the calls to the existing methods can be intercepted and manipulated.