Linux follows a different nomenclature for hard disks and partitions.
The first hard disk (master) is named /dev/hda for IDE drives, and /dev/sda for SCSI and SATA drives.
More often than not, as we have only one hard disk on our home PCs, this is the only one you'll have to deal with. If you have a second hard disk, it will be named as /dev/hdb
The entire linux filesystem is under one directory root (/) which includes subdirectories. The software installed and the hardware are grouped in these subdirectories. In linux everything is a file. You will get used-to this concept, with time.
So /dev is the directory under which drives are located (CD/DVD, floppy, hard disk).
Coming back to the nomenclature, the first partition on your hard disk is designated as /dev/hda1 (In Windows we would call it the C drive.), the second as /dev/sda2 (In Windows we would call it the D drive.), so on and so forth. Now everything is fine as long as you make all these partitions as primary partitions.
But hey!!!!!!! Wait a minute!!!!! Did you know, that you can have only FOUR primary partitions on a hard drive??? How do I partition my 160GB + hard drive??? I want more partitions!!!!! Yes you can have more partitions if you use logical partiitons instead of primary partitions. The first logical partition you make ALWAYS starts with the number 5 (so you can still accomodate 3 primary, get it
). The primary partition used to house the logical partitions is called an extended partition.
I think with this background you are now able to understand GParted.
For further reading I refer you to *tldp.org/HOWTO/html_single/Partition/
and *tldp.org documentation in general.