Primary Paritition>> Operating System will be install. We will create the four primary partition.
Then Extended Partition will be created. under these extended partition Logical partition will be create.
For Example If we create the extended partition up to 4GB.Under the 4GB we can create the Logical Partition.Like
LogVol00 / ext3
If we create the custom Layout it will create the partition like following
/dev/sda1 / ext3
/dev/sda2 /usr ext3
/dev/sda5 /disk3 ext3
/dev/sda6 /disk2 ext3
Here /dev/sda4 is Extended under these Extended we /dev/sda5,6 these two logical partition will be create.