What is the use of custom serialization ?where do we use it?how does it differ from other serializations?

It helps you, serialize the the custom object.Like while in developing if you want store the object(Employee) it will be serialized(lets say R1) and after some time lets assume you are adding an additional attributes and the latest object will saved(let say R2).Now if you open the R1 object in R2 it will thrown the exception here you need to implement the serialization which should support R1 and R2.