An extended partition is a partition that can be further divided to create additional partitions.
Basically, the extended partition allows you to have more partitions on a physical drive than you could otherwise have.
Here is an example of a disk with five partitions and a link to a definition of an extended partition.
http://www.info.org/extended_partition.html
$ df -a /dev/sda*
Filesystem 1K-blocks Used Available Use% Mounted on
udev 3910716 4 3910712 1% /dev
/dev/sda1 493235028 36226812 431930208 8% /
udev 3910716 4 3910712 1% /dev
/dev/sda5 151058636 107940272 35421980 76% /media/stephen/Hitachi72101Ptn5
udev 3910716 4 3910712 1% /dev
/dev/sda7 157554484 609108 148918960 1% /media/stephen/Hitachi72101Ptn7
/dev/sda8 151058636 60884 143301368 1% /media/stephen/Hitachi72101Ptn8