What is the Difference Between Log Off, Restart, and Shut Down?

To shut down (or power down, or power off) a system means that, when the process of shutting down is complete, the machine is powered down, and it will not come back up again without a someone taking further action, e.g., powering it back on again. If you expect your system will be needed for maintenance or remote access by someone else, shutting it down effectively prevents this.

To power on a system means that you simply press the power button and let the system come up to a login prompt.

If you enter your username and password, you have gone a step further and have logged on.

To restart (or reboot) a system means that the computer goes through a complete shutdown process, then starts back up again. This is frequently required after software installations or during troubleshooting.

To log off or Sign out a system means that the user who is currently logged on has their session end, but leaves the computer running for someone else to use. This is faster than a full restart and, generally, a better choice during the course of the business day when a system is shared between multiple users.

To Switch account means you will be leaving current user behind (not logged out) and attempt to login using your credentials. this will take lot of memory from the computer RAM. as a result computer will be very slow. also this will prevent others using the same documents opened in other users. so the best way is to sign out from the user before login to another.