Windows XP Standby versus Hibernate Mode?

Just wondering something, what exactly do each of these modes do? meaning do you really save that much electricity using either of these modes? I normally leave my computers running full time with no standby or hibernate. I was wondering if buy using standby I will save lots of money in electricity?

BTW, does anybody know how much in general a computer costs to run 24/7 per month?

Standby is an energy saving mode, meaning that the PC will switch the monitor and drives to a low power state. Hibernation on the other hand actually turns the machine off, but writes the contents of the physical RAM to a file called the hiberfil.sys, which is the same size as the amount of physical RAM your machine contains (if you have 256MB, then the file will be 256MB). When you turn the machine back on again, your programs and files will be exactly as you left them before you hibernated the system.

So when I use my laptop there usually isn't really any reason to turn it off instead of using hibernate?

Nope, you can even do the same on a desktop, saves masses of time on bootups.