Here are few commands which are commonly used in Linux for Starting & Stopping processes/systems:

shutdown
halt
reboot
startx

Shutdown brings the system down in a secure way. All logged-in users are notified that the system is going down, and login is blocked. It is possible to shut the system down immediately or after a specified delay

shutdown -h now

{Shutdown the system now and do not reboot}

Halt notes that the system is being brought down in the file /var/log/wtmp, and then either tells the kernel to halt, reboot or poweroff the system

halt

{Stop all processes - Same as above}

shutdown -r 5

{Shutdown the system in 5 minutes and reboot}

shutdown -r now

{Shutdown the system now and reboot}

Reboot is called when the system is not in runlevel 0 or 6, in other words when it's running normally, shutdown will be invoked instead (with the -h or -r flag)

reboot

{Stop all processes and then reboot – same as above}

startx

{Start the X system}