sleep just kind of puts your computer in heavy screen saver mode. You can set it up so that the monitor is powered down, the disks power down etc. Hibernate mode takes a snapshot of everything that is running and saves all that info as a file on your HDD and shuts down. When you come out of hibernation, instead of booting everything up, it just loads the hibernation info file and poof, you're right back where you were.
I think that reboot just reboots your OS from scratch, reset actually shuts down the hardware for a moment and is more like a cold boot up.
--- *warning* ---
I could be completely wrong
