That was me. By the letter of the law, standby is not power-off. Now, I'm not claiming you're going to bring down an airplane if your laptop is in standby mode, but standby mode is not off. Hibernate and off are really off.

In standby mode, the memory is kept warm, certain devices are kept warm, some in low power mode, some devices may be off, and the CPUs are on, but in very low power mode. The CPU has to be able to recognize when you open the laptop or move the mouse or hit a key in order to bring the laptop out of standby mode. (The devices basically send an interrupt to the CPU which will bring the CPU out of low power mode, so it can bring the rest of the system back up.)

When you hibernate, the contents of memory are written to disk, and all device state is saved to disk, and the power is turned off. When you turn the power back on, the firmware knows to resume from the saved state, restoring all devices to their previous state and restoring the contents of memory (that needed to be saved, not all memory pages need to be saved, for example, if they contain read only program instructions, they can be re-loaded from the program files as required .. aka on-demand) from the disk.

In standby mode, what's supposed to happen is that when the battery gets low enough (say 10% of capacity) it does a hibernate so you don't lose any work in progress when you went into standby mode. It has to power-up to do the hibernate so it can do that. You really don't want to disable hibernate mode if you just use standby mode. Of course, it is an option, so it's up to you if you want to do that. As Scott pointed out, you can set the amount of time that it will remain in standby mode, but if for some reason the battery gets down below the threshold before that timer expires, it will hibernate on its own.

My battery won't even hold a charge anymore, so I never use standby mode. I do power manage the display and the disks, but I'm always running it when connected to a power source.

-David