1) Difference between Serializable and Externalizable is that Serializable is a
marker interface i.e. does not contain any method but Externalizable interface
contains two methods writeExternal() and readExternal().
2) Responsibility of Serialization. when a class implements Serializable interface, default Serialization process gets kicked of and that takes responsibility of serializing super class state. When any class in Java implement java.io.Externalizable than its your responsibility to implement Serialization process i.e. preserving all important information.
3) This difference between Serializable and Externalizable is performance. You can not do much to improve performance of default serialization process except reducing number of fields to be serialized by using transient and static keyword but with Externalizable interface you have full control over Serialization process.
4) Difference between Serializable and Externalizable interface is maintenance. When your Java class implements Serializable interface its tied with default representation which is fragile and easily breakable if structure of class changes e.g. adding or removing field. By using java.io.Externalizable interface you can create your own custom binary format for your object.