When PC hard disks were invented people soon wanted to install multiple operating systems, even if their system had only one disk. So a mechanism was needed to divide a single physical disk into multiple logical disks. So that's what a partition is: A contiguous section of blocks on your hard disk that is treated like a completely seperate disk by most operating systems.
It is fairly clear that partitions must not overlap: An operating system will certainly not be pleased, if another operating system installed on the same machine were overwriting important information because of overlapping partitions. There should be no gap between adjacent partitions, too. While this constellation is not harmful, you are wasting precious disk space by leaving space between partitions.
A disk need not be partitioned completely. You may decide to leave some space at the end of your disk that is not assigned to any of your installed operating systems, yet. Later, when it is clear which installation is used by you most of the time, you can partition this left over space and put a file system on it.
Partitions can not be moved nor can they be resized without
destroying the file system contained in it. So repartitioning
usually involves backup and restore of all file systems touched
during the repartitioning. In fact it is fairly common to mess
up things completely during repartitioning, so you should back
up anything on any disk on that particular machine before even
touching things like fdisk.
Well, some partitions with certain file system types on them actually can be split into two without losing any data (if you are lucky). For example there is a program called "fips" for splitting MS-DOS partitions into two to make room for a Linux installation without having to reinstall MS-DOS. You are still not going to touch these things without carefully backing up everything on that machine, aren't you?
Tapes are your friend for backups. They are fast, reliable and easy to use, so you can make backups often, preferably automatically and without hassle.
Step on soapbox: And I am talking about real tapes, not that disk controller driven ftape crap. Consider buying SCSI: Linux does support SCSI natively. You don't need to load ASPI drivers, you are not losing precious HMA under Linux and once the SCSI host adapter is installed, you just attach additional disks, tapes and CD-ROMs to it. No more I/O addresses, IRQ juggling or Master/Slave and PIO-level matching.
Plus: Proper SCSI host adapters give you high I/O performance without much CPU load. Even under heavy disk activity you will experience good response times. If you are planning to use a Linux system as a major USENET news feed or if you are about to enter the ISP business, don't even think about deploying a system without SCSI. Climb of soapbox.
The number of partitions on an Intel based system was limited from the very beginning: The original partition table was installed as part of the boot sector and held space for only four partition entries. These partitions are now called primary partitions. When it became clear that people needed more partitions on their systems, logical partitions were invented. The number of logical partitions is not limited: Each logical partition contains a pointer to the next logical partition, so you can have a potentially unlimited chain of partition entries.
For compatibility reasons, the space occupied by all logical
partitions had to be accounted for. If you are using logical
partitions, one primary partition entry is marked as "extended
partition" and its starting and ending block mark the area
occupied by your logical partitions. This implies that the
space assigned to all logical partitions has to be contiguous.
There can be only one extended partition: no fdisk program
will create more than one extended partition.
Linux cannot handle more than a limited number of partitions per drive. So in Linux you have 4 primary partitions (3 of them useable, if you are using logical partitions) and at most 15 partitions altogether on an SCSI disk (63 altogether on an IDE disk).
In Linux, partitions are represented by device files. A device file is a file with type c (for "character" devices, devices that do not use the buffer cache) or b (for "block" devices, which go through the buffer cache). In Linux, all disks are represented as block devices only. Unlike other Unices, Linux does not offer "raw" character versions of disks and their partitions.
The only important thing with a device file are its major and minor device number, shown instead of the files size:
$ ls -l /dev/hda
brw-rw---- 1 root disk 3, 0 Jul 18 1994 /dev/hda
^ ^
| minor device number
major device number
When accessing a device file, the major number selects which
device driver is being called to perform the input/output
operation. This call is being done with the minor number as a
parameter and it is entirely up to the driver how the minor
number is being interpreted. The driver documentation usually
describes how the driver uses minor numbers. For IDE disks,
this documentation is in /usr/src/linux/Documentation/ide.txt.
For SCSI disks, one would expect such documentation in
/usr/src/linux/Documentation/scsi.txt, but it isn't there. One
has to look at the driver source to be sure
(/usr/src/linux/driver/scsi/sd.c:184-196). Fortunately, there
is Peter Anvin's list of device numbers and names in
/usr/src/linux/Documentation/devices.txt; see the entries for
block devices, major 3, 22, 33, 34 for IDE and major 8 for SCSI
disks. The major and minor numbers are a byte each and that is
why the number of partitions per disk is limited.
By convention device files have certain names and many system
programs have knowledge about these names compiled in. They
expect your IDE disks to be named /dev/hd* and your SCSI disks
to be named /dev/sd*. Disks are numbered a, b, c and so on, so
/dev/hda is your first IDE disk and /dev/sda is your first SCSI
disk. Both devices represent entire disks, starting at block
one. Writing to these devices with the wrong tools will
destroy the master boot loader and partition table on these
disks, rendering all data on this disk unusable or making your
system unbootable. Know what you are doing and, again, back up
before you do it.
Primary partitions on a disk are 1, 2, 3 and 4. So /dev/hda1 is
the first primary partition on the first IDE disk and so on.
Logical partitions have numbers 5 and up, so /dev/sdb5 is the
first logical partition on the second SCSI disk.
Each partition entry has a starting and an ending block address assigned to it and a type. The type is a numerical code (a byte) which designates a particular partition to a certain type of operating system. For the benefit of computing consultants partition type codes are not really unique, so there is always the probability of two operating systems using the same type code.
Linux reserves the type code 0x82 for swap partitions and 0x83 for "native" file systems (that's ext2 for almost all of you). The once popular, now outdated Linux/Minix file system used the type code 0x81 for partitions. OS/2 marks it's partitions with a 0x07 type and so does Windows NT's NTFS. MS-DOS allocates several type codes for its various flavors of FAT file systems: 0x01, 0x04 and 0x06 are known. DR-DOS used 0x81 to indicate protected FAT partitions, creating a type clash with Linux/Minix at that time, but neither Linux/Minix nor DR-DOS are widely used any more. The extended partition which is used as a container for logical partitions has a type of 0x05, by the way.
Partitions are created and deleted with the fdisk program.
Every self respecting operating system program comes with an
fdisk and traditionally it is even called fdisk (or FDISK.EXE)
in almost all OSes. Some fdisks, noteable the DOS one, are
somehow limited when they have to deal with other operating
systems partitions. Such limitations include the complete
inability to deal with anything with a foreign type code, the
inability to deal with cylinder numbers above 1024 and the
inability to create or even understand partitions that do not
end on a cylinder boundary. For example, the MS-DOS fdisk can't
delete NTFS partitions, the OS/2 fdisk has been known to
silently "correct" partitions created by the Linux fdisk that
do not end on a cylinder boundary and both, the DOS and the
OS/2 fdisk, have had problems with disks with more than 1024
cylinders (see the "large-disk" Mini-Howto for details on such
disks).