Neither audience nor the question is dumb.. some times it also means that ppl. are busy
Let me try to explain ..as far as I know.
To be frank, If you can understand the API doc of externalizable from JDK documentation, you are not a normal person .
Coming to the answer,
You must have noticed that Externalizable extends Serializable. Hence you can't put "VS" in between them. Externalizable is an enhancement over Serializable.
OK this is the theory behind Externalizable..
JVM has a mechanism to write or read an object from the I/O streams using its own representation (bytecode). This is done through readObject() and writeObject() APIs.
Assume if you want store the object in your own way, you must do with Externalizable.
It will look bit wiered, but thats how it is...
If you have Thinking in Java (Bruce Eckel), please refer it. The same example is available in this URL Java2S
Hope it helps.