Lock screen appears when you start or restart Windows 8 computer, or lock your computer by opening Start screen, clicking your user name and selecting Lock from the menu (keyboard shortcut Windows Key+L).
Lock screen covers the whole screen and displays current date and time, network and power status, plus notifications of Microsoft-provided Modern UI/Metro apps by default. To move on to user sign-in screen (aka Welcome screen), click or tap anywhere on Lock screen, or press a key on keyboard.
Each user can personalize his/her Lock screen, but Windows 8 displays the Lock screen of the user who was signed in last. For example, if both Margus and Mirjam are logged on to the same PC, but Mirjam locked her account later, then her Lock screen is displayed. The same applies to computer restarts.
Personalizing Lock screen in Windows 8
First, use keyboard shortcut Windows Key+I to open Settings charm, or move mouse pointer to the bottom right edge of screen and click Settings.
Click Change PC settings in the bottom part of the bar.
Touchscreen users should first swipe in from the right edge of screen and tap Settings.
PC settings Modern UI/Metro app opens in Personalize tab. On the right, Lock screen tab is already active.
First, you can change the background photo of your account's Lock screen. Either click any displayed photo or use the Browse button to find your own.
Browsing opens your Pictures library by default. Again, you can click any available photo and then click Choose picture to set it as your Lock screen background.
If you want to select a picture from another location, click the down arrow part of Files combo box and open any listed folder (you can also take a picture using your webcam or even use photos stored on your SkyDrive account).
Windows 8.1 (Blue) will add slide show capability to Lock Screen. It will support both locally stored photos and those stored on your personal SkyDrive account. Digital picture frame, anyone? 
Managing Lock screen apps and notifications
As noted before, Lock screen can display notifications from Modern UI/Metro apps - count of unread e-mails, upcoming appointments, etc. Let's see an example of this:
By default, notifications of Calendar, Mail, Messaging and Weather apps are displayed. Calendar displays a more detailed view as shown above (to the right of current time). Some people find the notifications and status updates to be a privacy threat, so they might want to customize or remove these apps from their Lock screens.
Apps that are already displaying status and notifications are listed in the Lock screen apps section of PC settings app, Personalize tab.
To add an application to Lock screen, click any available tile with + sign and then select the app you need. Please note that you can choose only these Modern UI/Metro apps that are capable of revealing their status and notifications.
To remove an app from Lock screen, click its tile and select Don't show quick status here. You can delete all apps if you want to.
Please note that if you remove the app that also displays detailed status and notifications (Calendar in this example), it will also be deleted from theChoose an app to display detailed status section.
To manage the single app that reveals more information, click its tile or plus sign in theChoose an app to display detailed status section.
Next, either click a listed app or the Don't show detailed status on the lock screen command.
Now you can close the PC settings Metro app using keyboard shortcut Alt+F4 or by moving mouse pointer to the top of screen where its pointer turns into hand and dragging the screen all the way down.
