Question: I'm really confused about these keywords used in Ubuntu. I've a little knowledge about suspend and hibernate in Ubuntu and how does it work. But I've no idea about what are the differences between sleep and standby. In result what are the differences between these four often used terms in Ubuntu?

I searched in Google before asking here but didn't get any answer specific to Ubuntu.

I'll be very happy to get a good definition, and appreciate for the time given to answer this question.

Answer: 
What's the difference between sleep, suspend, and hibernate? 

* Sleep (sometimes called Standby or “turn off display”) typically means that your computer and/or monitor are put into an idle, low power state. Depending on your operating system, sleep is sometimes used interchangeably with suspend (as is the case in Ubuntu based systems). What sleep does on your system should be evaluated on a case by case basis.
* Suspend saves its current state to your RAM and puts the computer and all peripherals on a low power consumption mode. If the battery runs out or the computer turns off for some reason, the current session and unsaved changes will be lost.
* When a computer hibernates (sometimes called suspend to disk), it will save its current state to the hard disk and power down completely. When resuming, the saved state is restored to RAM.
Hibernation stores all the necessary settings, registers, and RAM memory as a non-volatile magnetic trace to hard disk. The power can then be totally removed. {Plug withdrawn}. On restart everything is reset to how it was except the real-time. The software of some Apps may have to give special consideration to this.
S/by - preserves low voltage power to the RAM and such other registers as will be needed to be preserved. Power to the discs is removed after they are parked. {+/-12V} The power saving is considerable typically 95%. So if your PC was running 100W normally it will consume 5W in S/by or Sleep modes. In Hibernation mode it is (or can be) Zero.
BjG