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4 Ways To Boot an OS
   configfile,  symlinks,  direct booting,  chainloader

You can use these methods
--   in a boot menu
--   at a grub> prompt (at the GRUB CLI)
to boot OSs.  If you use them at the GRUB CLI, you must type the command boot at the end to make the methods symlinks, direct booting and chainloader work.  You do not need the command boot in a configfile menu.  You do not need the boot command in grub.cfg (it is implied there).

NOTE:  These are done the same as in GRUB Legacy but with a new format.
But see the note on configfile below.

NOTE:  Each boot entry starts with a line
menuentry "...some text..." {
and ends with
 }

>>>   Configfile booting
This is easy and flexible; however, theres a caveat:
Both OSs must use the same version of GRUB--both the OS you are working in (where the main boot menu is kept) and the OS you are booting.

For example, consider these two statements in a menuentry:
set root=(hd1,2)
configfile /boot/grub/grub.cfg

An equivalent way of doing it is this:
configfile  (hd1,2)/boot/grub/grub.cfg

That tells your GRUB 2 to access partition (hd1,2), find the file /boot/grub/grub.cfg, show you that file so you can make a choice of which OS to boot, then boot the OS you choose.  The GRUB doing the work is your native GRUB, the one you are using, not the GRUB in (hd1,2).  Your native GRUB must be able to interpret the commands it finds in the grub.cfg of the OS you wish to boot.

When to avoid configfile
When there is a mixture of GRUB versions on your PC, it is best to avoid using configfile.
Instead, use chainloader or symlinks.

Example (Taken from a grub.cfg.  The # sign indicates a comment.)

#  Kubuntu 8.04.3 on sdb2, by configfile
menuentry �Kubuntu 8.04.3 on sdb2, by configfile�  {
set root=(hd1,2)
configfile /boot/grub/grub.cfg
 }


>>>   Symlinks used in a boot entry
Symlinks are like shortcuts, and they direct control from the link to the file they point at.  Symlinks for the kernel and initrd files are already set up for you.  Look for yourself.  List all files (-a) in long or detailed form (-l) under your root partition (/) by issuing the following command at Konsole:

ls -a -l  /
lrwxrwxrwx   1 root root    33 2009-08-02 11:01 initrd.img -> boot/initrd.img-2.6.24-24-generic
lrwxrwxrwx   1 root root    33 2009-04-19 17:14 initrd.img.old -> boot/initrd.img-2.6.24-23-generic
lrwxrwxrwx   1 root root    30 2009-08-02 11:01 vmlinuz -> boot/vmlinuz-2.6.24-2 4-generic
lrwxrwxrwx   1 root root    30 2009-04-19 17:14 vmlinuz.old -> boot/vmlinuz-2.6.24-23-generic

The symlink for the kernel is vmlinuz, and vmlinuz points at the NEWEST kernel.
The symlink for the initrd is initrd.img, and initrd.img points at the NEWEST initrd.

NOTE:   The "l" at the left indicates "link."  Note how the links point at their target files following the symbol ->.  Note that the symlinks are stored in your OS at the root level.

NOTE:   If you wish to make a boot entry for the older kernel and initrd, you must use the symlinks for them, vmlinuz.old and initrd.img.old.

If you use direct booting, you have to write out the exact kernel and initrd names:
linux /boot/vmlinuz-2.6.24-24-generic root=UUID= vb687f89-ggjd-6ach-7755-77241i4b4fk3 ro quiet splash
initrd /boot/initrd.img-2.6.24-24-generic

With symlinks, vmlinuz starts at root and points at  vmlinuz-2.6.24-24-generic  and  initrd.img  starts at root and points at  initrd.img-2.6.24-24-generic, so you can write more simply,
linux /vmlinuz  root=UUID= vb687f89-ggjd-6ach-7755-77241i4b4fk3 ro quiet splash
initrd /initrd.img

However, to simplify this when you are in an emergency trying to boot at the GRUB prompt (grub>), do it this way:
Suppose this OS is Kubuntu 9.10 in (hd0,8) = sda8.  Then youd have

menuentry �Kubuntu 9.10 on sda8, by symlinks�  {
set root=(hd0,8)
linux /vmlinuz root=/dev/sda8 ro quiet splash
initrd /initrd.img
}

or even simpler, when you cant remember the kernel options  "ro quiet splash":

menuentry �Kubuntu 9.10 on sda8, by symlinks�  {
set root=(hd0,8)
linux /vmlinuz root=/dev/sda8
initrd /initrd.img
}

(ro = Read-Only and is the default.  Thanks to dibl for this information.)

NOTE about the paths
Notice the paths to the symlinks start at root / (since the symlinks are stored under root /); whereas the paths to the actual kernel & initrd files start at /boot (since the kernel and initrd are stored under /boot)

Another Example

# Kubuntu 9.10 on sdb7 symlinks, normal mode
menuentry �Kubuntu 9.10 on sdb7, by symlinks�  {
set root=(hd1,7)
linux /vmlinuz root=/dev/sdb7 ro quiet splash
initrd /initrd.img
}

This is normal mode.  If you want recovery mode (single user),
replace the kernel options �ro quiet splash� with �ro single� (without quotes)


>>>   Direct booting
In direct booting, you use the actual kernel and initrd file names in the boot menuentry.
NOTE
If you use direct booting at the GRUB CLI, ie., at grub> prompt, you may use TAB completion to make your job easier.
For example,
grub>linux /boot/<Press TAB key now>
and you will get some choices, such as vmlinuz-2.6.24-24-generic

Example

#   Kubuntu 8.04 on sdb3, direct booting
menuentry "Kubuntu 8.04 on sdb3, direct booting" {
set root=(hd1,3)
linux   /boot/vmlinuz-2.6.24-24-generic root=UUID=db287e84-cbdd-4ca1-8745-85241a3b3fe2 ro quiet splash
initrd   /boot/initrd.img-2.6.24-24-generic
}


>>>   Chainloader booting
When you boot an OS by chainloading, you use the chainloader command to pass control of the booting from your native GRUB to some other bootloader located in the MBR of a disk somewhere or in the
boot sector of a partition.

NOTE:  For this to work, a bootloader must first be installed to the target MBR or a partition boot sector.

A Linux example

menuentry �Kubuntu 9.10 on sdb7, by chainloader�  {
set root=(hd1,7)
chainloader +1
}

"chainloader +1" says to go to sector 1 (of (hd1,7)) and turn control over to the bootloader found there.

For this example to work, a bootloader (e.g., GRUB 2) must be installed to the partition (hd1,7).
To install GRUB 2 to the boot sector of partition sdb7 (=(hd1,7)):
sudo grub-install /dev/sdb7

This is another way to write the same boot entry:
menuentry �Kubuntu 9.10 on sdb7, by chainloader�  {
chainloader  (hd1,7)+1
}

Another Linux example

menuentry �Drive sdc = hd2 by chainloader�  {
chainloader  (hd2)+1
}

If a bootloader (e.g., GRUB 2) has been installed to the Master Boot Record of drive sdc (= hd2), the menuentry will boot that hard drive by turning control over to the bootloader in its MBR.

For this example to work, a bootloader (e.g., GRUB 2) must be installed to the Master Boot Record of drive sdc (= hd2).  To install GRUB 2 to the Master Boot Record of drive sdc:
sudo grub-install /dev/sdc

Example

Windows XP on sda1
menuentry �Windows XP on sda1, by chainloader�  {
set root=(hd0,1)
chainloader +1
}