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.
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)