Q:

What is the difference between $ sudo reboot and $ reboot?

I have got Raspberry Pi 2 Model B v1.1 with NOOBS v1.4.2 Raspbian loaded. 
What is the difference between 
$ sudo reboot 

and 
$ reboot 

typing in terminal?

A:

Adding sudo before a command runs the command as if you were the user root. Some commands require to be run as root because it is the highest user by default. When you run sudo reboot you are running reboot from root. If you just type reboot you are running reboot from your current user, which may not have the necessary permissions to enter system commands like that. 

