just as you can mount devices onto the file system heirarchy after boot-up, during booting the partitions containing the basic heirarchy (/) are also mounted.
at some stage after the kernel is started at boot time, the /etc/fstab is read. this file contains information on which partition is to be mounted on which mount point. a mount point is simply the directory in the entire file system. the listing on my setup is like
LABEL=/ < space> / < space> ext3 < space> defaults < space> 1 < space> 1
LABEL=/boot < space> /boot < space> ext3 < space> defaults < space> 1 < space> 2
/dev/hda6 < space> /tmp/d < space> vfat < space> defaults < space> 1 < space> 2
#replace < space> with actual spaces
the explanation from the man pages...
The first field, (fs_spec), describes the block special device or remote filesystem to be mounted.
The second field, (fs_file), describes the mount point for the filesystem.
The third field, (fs_vfstype), describes the type of the filesystem. Linux supports lots of filesystem types, such as ext2, ext3, hfs, hpfs, iso9660, jfs, minix, msdos, ntfs, proc, reiserfs, vfat, xenix, xfs, and possibly others.
The fourth field, (fs_mntops), describes the mount options associated with the filesystem.
It is formatted as a comma separated list of options. It contains at least the type of mount plus any additional options appropriate to the filesystem type.
The fifth field, (fs_freq), is used for these filesystems by the dump command to determine which filesystems need to be dumped. If the fifth field is not present, a value of zero is returned and dump will assume that the filesystem does not need to be dumped.
The sixth field, (fs_passno), is used by the fsck program to determine the order in which filesystem checks are done at reboot time. The root filesystem should be specified with a fs_passno of 1, and other filesystems should have a fs_passno of 2.
the label / and /boot i think are defined in the boot loader config file. if you notice the third line in my config, /dev/hda6 type vfat, its my common windows fat32 partition where i store all my data / articles etc.