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Its an alternative way of extending the behaviour of a Class. Usually we would inherit a Class and over-ride its methods but this pattern tells us how to extend the behaviour by using composition.
A good example is the Java Buffered IO classes. It extend the behaviour of IO writter classes by adding buffering behaviour. It means ecah time you call write(..) fucntion of a Buffered IO Object, it will not write to disk immediately but the data will be stored in memory and after some threshold is reached it will be written to disk. This way effeciency of IO opertions is increased.
So we could have inherited the IO writers to achieve it but instead in Java they have used Decorator pattern. The Buffered class which is the decorator contians a instance variable of the IO class type which it is decorating. When write(..) of Decorator is called, it will keep on using memory biffer and finally it will call the write(..) of the contained IO Object.
Reason for choosing this approach can be that we do not want to create a complex inheritence tree, each time we need a new functionality. As in Java Multiple inheritence is not allowed so its a good way of filling that gap.
