a HD can have 4 Pri. or 3 Pri and one Ext. partitions max. if you make one of them ( 4 ) extended, you can make a lot ( i think upto 60 ) logical stations in that extended part.
so an Ext. part. is useless untill you make a logical station in that partition
( if hda4=Ext then the first usable logical part. will be hda5 )
( God knows why...