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, 8/8.1, and 10, so let's see some examples.
In Windows 10, Lock Screen appears first. As in Windows 8 and 8.1, it displays the current time and date, plus notifications from selected Windows Store apps for the user that signed in last. Network and Power status icons are on the right now.
Click or touch anywhere on the screen, or press a keyboard key to reveal the sign-in screen.
The welcome or sign-in screen displays the last user who signed in to the device. Other available user accounts are listed on the left bottom of the screen. The right side of the screen shows interactive icons for regional settings and keyboard layout, network, accessibility, and power.
Since Windows 10 Anniversary Update, you can hide account details (e-mail address) from the Sign-in screen by opening the Settings app, navigating to Accounts, Sign-in options, and setting the Privacy or Show account details slider to off.
In Windows 8 and 8.1, Lock Screen also appears first. It displays the current time and date, plus some informational icons about network and power status (the latter appears on laptops and tablets only). Some Windows Store (aka Modern UI or Metro) apps, such as Calendar and Mail display notifications for the user that last signed in. No user accounts are displayed on this screen. To sign in, you must either click anywhere on the screen or press a keyboard key once. Touch screen users should slide up the screen with their finger or pen.
Lock Screen slides up and reveals the last user who logged on to the device. To see all available accounts on the computer, click the Back button (the ring with an arrow pointing to the left).
On the bottom left, there is a button for Accessibility options. On the bottom right, there are the current region (top and in bold) and keyboard setting indicators and the Power button.
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 the Ease of Access (Accessibility) options.
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 or restarting the computer. Click an account to log on.
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 needs quick access to his/her files or e-mails, the quickest way is to use the keyboard shortcut Windows Key+L to lock your Windows session and allow others to log on.
You can also use the Start menu or Start screen for this: click the Start button (or Start tip in Windows 8 and 8.1) on the bottom left or press the Windows key once.
In the original version of Windows 10, click your user name on the top left of the Start menu and select another user account from the bottom of the list. This will lock your session and open the sign-in screen for the selected account.
In Windows 10 Anniversary Update and newer, you only see your user account picture on the left side of the Start menu (fourth icon from the bottom by default, excluding Start). Click or touch it and choose another account.
In Windows 8 and 8.1, click your user name on the top right and select another user account from the bottom of the list. This will lock your session and open the sign-in prompt for the selected account right away.
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 XP, click the Log Off button.
Then click Switch User in the Log Off Windows menu.
Logged-on users are easily distinguishable until Windows 10. Windows XP even displays the number of running programs for the user(s), while Windows Vista and 7 just display the "Logged on" line. Windows 8 and 8.1 state "Signed in" for active users. Windows 10 is the only version that does not reveal which user has already signed in.
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, reboots the device, and starts Windows again. You will be warned if some other user account is also logged on to the computer.
Install updates and restart in Windows 8 and newer means that Windows closes all open programs and documents, logs all users off, installs updates, reboots the device, and starts Windows 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 device. To start your computer again, you will have to press your computer's power button. You will be warned in case some other user is also logged on to the computer.
Install updates and shut down in Windows 10 means that Windows closes all open programs and documents, logs all users off, fully installs updates, and then completely turns off your device. As the installing of updates is a two-stage process since Windows Vista, most Windows 10 versions do the first part of the installation, then restart your PC, finish the update process, and only then shut down your device. This could be a tiny problem when your PC's system drive is encrypted with BitLocker, VeraCrypt, or some other encryption software: if a password or a PIN is required to start a computer, this prompt stays on instead of actually finishing the installation process.
All modern laptops, desktops, and tablets support two more options - Sleep/Standby and Hibernation.
Sleep or Standby means that the device turns almost all its power off to save battery or energy. All programs and apps are in a paused state and the computer keeps on running at a very low voltage. You can resume from standby by pressing your computer's power button once and you will have all your running programs back on your display within a few seconds.
Most laptops go to sleep automatically when you close the lid and wake up after reopening the lid.
If your laptop's or tablet's battery runs out during standby, or a power outage occurs on a desktop computer, you will lose all information about running programs. This means that all unsaved changes in open documents are lost. Modern laptops and tablets shut down automatically if the battery gets to a critical level.
Hibernate or hibernation means that all information about running programs and apps in the device's random access memory (RAM) is written to a hibernation file on the hard disk drive. After this process is complete, the device turns off completely. Turning the device back on again makes Windows copy the information back to the device's memory and you will have all your running programs back on your display within a few minutes.
Hibernation is a longer process than sleep/standby, but as your device will be turned off completely, it is not in danger of losing information in case of a dead battery or a power outage.
Because writing large amounts of data to a hibernation file can wear down Solid State Drives (SSD-s) quickly, hibernation is disabled by default since Windows 7. You should only enable this power feature if Windows is installed on a traditional hard drive or a hybrid drive.
To access these options in Windows XP, Vista, 7, and 10, open the Start menu by clicking the 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 the original Windows 10, click or touch Power on the bottom of the Start menu and choose the option you need.
Windows 10 Anniversary Update and newer remove the text, so click or tap the Power icon/button and select what to do next.
In Windows 8, either move the mouse pointer to the lower-right edge of the screen and click Settings; or use the keyboard shortcut Windows Key+I to open Settings Charm.
Click or tap the Power button and select the option you like.
In Windows 8.1, you can either use the keyboard shortcut Windows Key+X to open the Quick Links menu, or you can right-click or touch and hold the Start tip on Taskbar to access the Shut down or sign out submenu. The Quick Links menu method also works in Windows 10.
Windows 8.1 Update (available since 8th of April, 2014) adds power options button to Start screen on desktop and laptop computers (but not on tablets), right next to your profile picture.
The Settings Charm method from Windows 8 still works, too.
In Windows Vista, there are two default buttons available besides the full Shutdown options menu. The first one is normally Shut down (but it can be changed in Start menu options) and the second one 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 the Start menu and Power options). Clicking the button performs the action without any confirmation.
Full shutdown options are available by clicking the button with an arrowhead pointing to the right. Again, all actions are performed without confirmation.
In Windows XP, click Turn Off Computer.
Then click the option you need in the Turn off computer dialog.
In Windows 8, 8.1, and 10, you can quickly boot into the Advanced Startup menu by holding down the Shift key while clicking or tapping Restart. This menu allows access to many troubleshooting tools.
Shutdown options are also available on the Welcome Screen/Sign-in screen (see the beginning of the article).