What is the main difference between reboot and restart of a machine

What is the major difference between reboot and restart of operating system? Is it just terminologies?

Any technical background process evolving around during reboot and restart of operating system?

It is indeed a terminology issue.

Reboot is more specific, and involves the computer reloading the boot loader on the boot drive. Some operating systems "reboot" by an ACPI command, which "restarts" the computer.

Restart is vague, and can mean the same as reboot, or a reload of the current operating system (without the boot loader), or even just restarting the user mode part of the operating system, leaving the kernel mode memory intact. It may also mean allowing the BIOS to reselect the boot drive and boot loader, which may require resetting the motherboard by way of an ACPI command.