After your Windows computer starts, you will most probably see a Welcome screen or Lock screen. Here you can click your user name and enter your password to log in to Windows.
The screen is a little different in Windows XP, Vista, 7 and 8/8.1, so let's see some examples.
In Windows XP, the Welcome screen looks like this. User accounts are listed from top to bottom and on the bottom left there are options for turning off the computer. Click an account to log in.
In Windows Vista and 7, user accounts are listed from left to right and shutdown options are on the bottom right. On the bottom left, there are Ease of Access (Accessibility) options.
In Windows 8 and 8.1, Welcome Screen has been renamed to Lock Screen. It displays current time and date, plus some informational icons about network and power status. Some Modern UI/Metro apps, such as Calendar and Mail can display notifications here. No user accounts are displayed. To log in, you must either click anywhere on the screen, or press any keyboard key once.
The cover screen slides up and reveals the last user who was logged on. To see all available accounts on the computer, click the Back button (the ring with an arrow pointing to the left).
On the left bottom there is a button for Accessibility options. On the right bottom there are current region (top and in bold) and keyboard setting indicators and Power button.
Since Windows XP, users do not need to close running programs and log off completely to allow some other user to log on. In case someone need quick access to his/her files or e-mails, use keyboard shortcut Windows Key+L to lock your Windows session and allow others to log on.
In Windows XP, Vista and 7, you can also use Start menu for this. Open it by clicking Start button on the bottom left or by using keyboard shortcut Ctrl+Esc or just by pressing Windows key once.
In Windows XP, click the Log Off button.
Then click Switch User in the Log Off Windows menu.
In Windows Vista and 7, click the Shutdown options button (the arrowhead pointing to the right) and select Switch User from the menu.
In Windows 8 and 8.1, press Windows key to open Modern UI/Metro-style Start screen, click your user name on top right and select another user account from the bottom of the list. This will open a logon prompt for the selected account right away.
In Welcome Screen, any logged on user is easily distinguishable. Windows XP even displays the number of running programs for the user(s), while Windows Vista and 7 just display "Logged on". Windows 8 and 8.1 state "Signed in" for active users.
To log back on, click your user name and enter your password.
Restart means that Windows closes all open programs and documents, logs all users off and then reboots the PC and starts Windows, so that you will see Welcome screen again.
Turn off or Shut down means that Windows closes all open programs and documents, logs all users off and then completely turns off your computer. To start your computer again, you will have to press your computer's power button.
Any modern laptop or desktop computer supports two more options - Sleep/Standby and Hibernate.
Sleep or Standby means that computer turns almost all its power off to save battery or energy. This does not mean that the PC is completely turned off - all programs are temporarily stopped and the computer keeps on running at a very low voltage. You can resume from standby by pressing computer's power button once and you will have all your running programs back on your monitor within a few seconds.
If your computer's battery runs out during standby or power outage occurs on desktop computer, you will lose all information about running programs and this can potentially damage your computer's hard disk drive (HDD, aka hard disk) or information on it.
Hibernate or hibernation means that all information about running programs in computer's random access memory is written to a special file on computer's hard disk drive and after that process is complete, computer will be turned off completely. After you press power button again, all information in the special hibernation file is written back to computer's memory and you will have all your running programs back on your monitor within a few minutes.
Hibernation is a longer process than stand by, but as your computer will be turned off completely, you are not in danger of losing information or damaging components during power outage.
To access these options in Windows XP, Vista and 7, open Windows Start menu by clicking Start button, pressing Windows Key or using the Ctrl+Esc keyboard shortcut.
Please note that the images were taken on a virtual machine that did not support Sleep/Standby or Hibernate options.
In Windows XP, click Turn Off Computer.
Then click the option you need in the Turn off computer dialog.
In Windows Vista, there are two default buttons available besides the full Shutdown options menu. The first one is normally Shut down (but it is changeable in Start menu options) and the second allows to Lock your computer and switch users.
Clicking either of the buttons performs the action without any confirmation.
In Windows 7, the default button is Shut down (also changeable in Start menu and Power options). Clicking the button performs the action without any confirmation.
Full shutdown options are available by clicking the button with arrowhead pointing to the right. Again, any action is performed without confirmation.
In Windows 8, either move mouse pointer to the lower right edge of the screen and click Settings; or use keyboard shortcut Windows Key+I to open Settings Charm.
Click or tap Power button and select the option you like.
In Windows 8.1, you can either use keyboard shortcut Windows Key+X to open Quick Links, or you can right-click Start "tip" on Taskbar to access the Shut down or sign out submenu.
Windows 8.1 Update (available since 8th of April, 2014) adds power options button to Start screen, right next to your profile picture.
The Settings Charm method from Windows 8 still works, too.
Shutdown options are also available on Welcome Screen/Lock Screen (see the beginning of the article).