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: 
Saurav. I am answering this under my knowledge. Ubuntu dont have separate things suspend & sleep. When ever a Ubuntu system getting to be on suspend Ubuntu uses `/etc/acpi/sleep.sh` to enter/leave suspend mode. So that name indicating both suspend & sleep are calling the same script and they will have equal functionality.

If you want to know about what task will take care at the time of suspend/sleep means please read here : [Wiki Ubuntu Community](https://wiki.ubuntu.com/UnderstandingSuspend) 

Usually Hibernation means we know that it will saves the current state of the system. 

Officially 

> 
> When a computer hibernates, it will save its current state to the hard
>  disk and power down completely. When next the computer boots, the
>  prior state is restored just as you left it.
> 
> 
> 

For more about Hibernation : [Wiki Ubuntu Community](https://help.ubuntu.com/community/PowerManagement/Hibernate)
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 {Plug withdrawn}.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