Hey Sammy777,

I remember being puzzled by the same question:)

Think about it as having a physical server.

You can log in to the OS and issue shut down from the OS interface. - That is Guest Shutdown. The tasks are ended, information stored. RAM cleared in a safe fashion with preserving the data that is required.

You can tell OS to reboot. That is Guest Reboot.

You can push the power button on the server and force it to shut down. Or even pull the power cord. That would be the effect of the power off. Not recommended unless a last resort option.

Power cycle is a combination of a power off followed by a power on.

Reset is equivalent to holding the power button pressed forcing the host to reboot. Not as clean as when performed from the guest OS.

I guess the takeaway would be to use Guest operations where possible. And do not use power off unless you have working access to IPMI (not so much applicable to VMs, of course).

Let me know if this helps or if you have furhter questions.

Thanks a lot!
@Alona 

This was enlightening.