fstab stand's for File System TABle. It is where the system administrator can tell the OS about any filesystems the machine may have access to. It also allows default parameters to be provided for each filesystem.

A typical fstab looks something like the following:

#
# /etc/fstab
#
# <device>      <mountpoint>    <filesystemtype><options>  <dump> <fsckorder>

/dev/hdb5	/               ext2     	defaults       	1 	1
/dev/hdb2	/home        	ext2     	defaults       	1 	2
/dev/hdc	/mnt/cdrom   	iso9660  	noauto,ro,user 	0 	0
/dev/hda1	/mnt/dos/c   	msdos    	defaults       	0 	0
/dev/hdb1	/mnt/dos/d   	msdos    	defaults       	0 	0
/dev/fd0	/mnt/floppy  	ext2     	noauto,user    	0 	0
/dev/hdb4	none         	ignore   	defaults       	0 	0

none     	/proc        	proc     	defaults
/dev/hdb3	none         	swap     	sw

Note that this system has two IDE partitions, one which is used as /, and the other used as /home. It also has two DOS partitions which are mounted under /mnt. Note the user option provided for the cdrom, and the floppy drive. This is one of the many default parameters you can specify. In this case it means that any user can mount a cdrom, or floppy disk. Other options will be dealt with later.

fstab consists of a number of lines (one for each filesystem) seperated into six fields. Each field is seperated from the next by whitespace (spaces/tabs).

So from the example given previously:

/dev/hdc	/mnt/cdrom   	iso9660  	noauto,ro,user 	0 	0

The first field (/dev/hdc) is the physical device/remote filesystem which is to be described.

The second field (/mnt/cdrom) specifies the mount point where the filesystem will be mounted.

The third field (iso9660) is the type of filesystem on the device from the first field.

The fourth field (noauto,ro,user) is a (default) list of options which mount should use when mounting the filesystem.

The fifth field (0) is used by dump (a backup utility) to decide if a filesystem should be backed up. If zero then dump will ignore that filesystem. The sixth field (0) is used by fsck (the filesystem check utility) to determine the order in which filesystems should be checked.
If zero then fsck won't check the filesystem.
(as the example line above is a cdrom there is very little point in doing a fsck on it, so the value is zero).