Externalizable is subclass of Serializable. readExternal(ObjectInput), writeExternal(ObjectOutput) are two methods that should be implemented by classes implementing Externalizable interface. Its the responsibility of the class to save/restore contents of the object/super-types etc.
Serializable uses reflexion to marshal/un-marshal object. Classes implementing Serializable interfaces dont have to implement any methods for saving/restoring contents of objects.
One argument is that Externalizable is so java 1.1, java reflexion is improved a lot and we dont need Externalizable these days as performance savings due to Externalizable is minimal. This is NOT correct. Look at https://github.com/eishay/jvm-serializers/wiki/ which compares performance of various java serializers. You can see that java manual serialization (using externalizable) is 50X faster than java built-in serialization.
Links:
http://download.oracle.com/javase/1.3/docs/api/java/io/Externalizable.html
http://geekexplains.blogspot.com/2008/05/externalizable-in-java-what-is-it-used.html
http://stackoverflow.com/questions/817853/what-is-the-difference-between-serializable-and-externalizable-in-java
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