What is Difference between shutdown (power off) and restart (reboot) ?
What type of signals passing at time of reboot and poweroff, to where ?
What is difference between these two signals ?
At the time of reboot,
Is the operating system go to shutdown mode ?
If it goes shutdown, Which is make it to start again ?
Please correct it, if you find and mistakes in my questions
For the operating system, shutdown and reboot are practically the same thing.
The OS will shut down as usual in both cases. The only difference is the signal sent to the hardware at the very last moment.
To switch off/reboot the computer, the OS will send a special signal to the hardware once it is finished shutting down. It's only this signal which will be different between a reboot and a shutdown.
The way the signal is sent depends on the exact hardware platform. On modern systems it is usually done via ACPI.
The difference is that during reboot system is entering runlevel 6 and for shutdown it is runlevel 0.
It is similar to running
init <runlevel_number>
For more details on runlevels see http://en.wikipedia.org/wiki/Runlevels
Under /etc/rc<runlevel_number>.d/ (e.g. /etc/rc6.d/) directories you will find script executed on entering specific runlevel.
