The earliest personal computer operating systems were designed so that no ``damage'' would be caused if a floppy disk was abruptly ejected, or the machine shut off. UNIX does NOT have this property! UNIX does not write everything to disk when you tell it to, rather it keeps copies of things in memory under the idea that you might need it again soon. Because of this, storage devices need to be ``mount''ed and the computer as a whole needs to be told you want to shut it down, so that it can do some housekeeping first.
If you want to turn the power off, the command you need is shutdown. And typically you are wanting to do this ``now'', but you can specify a time to wait before killing everything. If you want to turn the power off, you need to use the ``-h'' switch to halt the machine (or you could use the halt command). If you want to reboot (or boot another operating system), you could use the ``-r'' switch (or the reboot command).