We use these terms log off, shut down, restart on a daily basis, especially when you call the customer care to troubleshoot a puzzling issue with your computer or mobile devices. But, for a layman these words may sound perplexing. So, lets get started to lessen the oblivion and understand the differences.
To log off a computer means to end the currently logged in session to make it available for other user who may have his/her own account or continue to use admin account, whichever user has access to while the system is still in running mode.
Shut down terminates all the running processes and services completely. Even the power of the computer is turned down. Shut down performs POST (Power On Self Test) operation which is primarily to verify whether the hardware is working as per expectation, before handing control to the system to load OS.
Restart stops processes relevant to OS, while hardware- CPU, RAM are still running and powers on the computer on its own. Unlike shut down, it does not perform POST. Restart is required specifically during software installation/uninstallation and troubleshooting.