I read an interesting technet article today about the difference between Sleep and Hibernate on Windows Vista computers. I'd never really given it much thought before but the article makes perfect sense. I've distilled the key points if you'd prefer to read on.
Sleep is a low power state that preserves whatever you were doing in memory. A sleeping computer can still wake up at specified times to download and install updates and perform other routine maintenance tasks. After these tasks have finished, the computer will go back to sleep automatically.
Hibernate is a power saving feature that writes your settings and memory content to disk before completely powering down the system. Waking from a state of hibernation takes longer than waking from sleep. This is because your computer needs to load your data and applications from the hard drive back into memory. Hibernate is particularly useful on a laptop when you know you won't be using the computer for an extended period of time and you won't have an opportunity to charge the battery.
Screen savers are apparently antiquated and do not conserve energy. In fact, many of today's sophisticated screen savers use more energy than your computer would use under light conditions. Even when a display goes blank, many screen savers continue to run and consume energy. And some screen savers actually prevent your PC from going to sleep.