Sleeping is a good way to pause, but it still uses power. Hibernate mode lets you save the contents of memory to disk so you can resume later on.
You learned how Ubuntu supports ACPI sleep. However, because ACPI sleep does require a trickle of power to keep the CPU and RAM alive, it may not be desirable in all situations. Hibernate has been designed as the complement to ACPI sleep. It takes the contents of RAM and saves them to the system's swap partition. As a result, it requires absolutely zero power to maintain that hibernated state. There is a downside to hibernating a machine, however. Due to the fact that the system saves the contents of RAM before powering off, and then loads the contents of RAM after the kernel loads on reboot, it does take a fair amount of time to enter and exit the hibernated state. However, hibernating is still faster than powering off and restarting your machine, and there's the added benefit of saving application state.
Since hibernation saves the exact contents of RAM to your swap partition, the size of your swap partition must be larger than the amount of RAM you have in the machine. Ubuntu configures this automatically if you have done a default installation, but if you override the default partitioner during installation, you need to keep this point in mind.
Configuring Hibernation
Since hibernation and sleep are similar and use the underlying Linux ACPI subsystem, they both use gnome-power-manager to configure and control their settings. You can adjust the default type of sleep to be hibernation from within the preferences of gnome-power-manager, but keeping the default set to Suspend makes sense due to the time involved in entering and exiting a hibernated state. If you do decide to adjust this setting, you can right-click on the gnome-power-manager icon, select Preferences, and click on the Options tab
Using Hibernation
Now that you have your preferences set how you like them, it's time to test hibernation. Assuming you're using the stock Ubuntu preferences for gnome-power-manager, you'll need to engage hibernation by right-clicking on the gnome-power-manager applet and selecting Hibernate. Once you do, the system will immediately dim the screen, and you should hear a good deal of hard-disk activity. Once the disk stops churning, the system will power off. At this point, the system is in hibernate mode and can be left in this state indefinitely without using any battery power. To exit the hibernated state, simply power on the system normally. The bootloader will come up, and the kernel will boot normally, until it detects a RAM image on the swap partition. At that point, the system will load up the RAM image and should return to where you left it. Typical times to enter hibernation run between 30 seconds and 1 minute, and times to exit hibernation (including the BIOS test) run about the same. Between sleep and hibernation, you have all the great power management capabilities at your disposal.
If your system is configured to boot multiple operating systems, you need to be careful here. In theory, you should be able to hibernate your Ubuntu system and then boot into a different operating system. But this is fundamentally risky: if you change anything on the Ubuntu partition, you'll be in heaps of trouble. And if you're sharing a swap partition between Ubuntu and another Linux distribution, you'll be in a world of trouble if that other Linux distribution boots up, since it will erase your hibernated state (or may itself try to resume from that hibernated state). Play it safe: if you are hibernated, don't boot into anything except the system you hibernated from.