idoc exetension is nothing but a to add the some more
functinality to the basic idoc. that is called the idoc
exetension. this is done by using the transaction code we30.
and go select the radiobutton like "exetension".

exextened functionality and use more functionality to the 
basic idoc is nothing but idoc extention.

Idoc extension is nothing but adding extra functionality to 
the existing message type.

Steps : 

1.Go to Tcode WE30.
   In that click the radiobutton extended.Execute and save.
2.Go to Tcode we20 (create patner profiles).
   in this after entering the patner profile and when u 
press the enter button then it will direct u to another 
page wherein u enter the message type and if it is the 
extended idoc then extended message type also need to 
specified.

Adding extra functionality to basic idoc is the concept of 
extended idoc.
go to we 30 click on extension idoc radio button

idoc is nothing but intermediatery documentation for inter 
company posting purpose its used