Disk are now huge, 13 Gb are not rare, so that it's not really handy to have all this stuff packed in only one part. Only windows do so, and if you use Linux, may be it's because you are aware of how inefficient the other is.
So a hard disk is usually cut in some pieces called "partitions" (see the "partition mini HOWTO" for details, also read /usr/doc/package/util/README.fdisk)
Let's get a look at (part of) my own print of fdisk -l :
Disk /dev/hdb: 255 heads, 63 sectors, 523 cylinders Units = cylinders of 16065 * 512 bytes
Device Boot Start End Blocks Id System
/dev/hdb1 1 153 1228941 83 Linux
/dev/hdb2 154 166 104422+ 82 Linux swap
/dev/hdb3 * 167 291 1004062+ 83 Linux
/dev/hdb4 295 523 1839442+ 5 Extended
/dev/hdb5 295 422 1028128+ 83 Linux
/dev/hdb6 423 523 811251 6 FAT16
This is my second hard disk, tied to guesses and tries (the first is too simple to be interesting).
/dev/hdb is my second ide disk (slave on the primary interface),
/dev/hdb1 is the first primary partition, running from the first (1) block to the block 153.
There can be four of such primary partitions. If one wants more than 4, one of them must be an "extended" one (not necessarily the fourth) and all other partitions are named "logical" and are located inside the extended one. Notice that partition number 5 and partition number 4 have the same beginning. Number five is logical, number 4 extended. Logicals begin always at 5, even if there are not 4 primary ones.