The Start screen is the most obvious new feature in Windows and it enables you to run tablet-like full apps like News, Weather, Travel, People and thousands of apps from the Store. Unlike desktop programs, they don't have window controls or regular menus. It therefore isn't obvious how you close them when you have finished using them. Here's what to do.
Open a few apps and then move the mouse to the top left corner of the screen. The last app used is displayed as a thumbnail image and moving the mouse down the left edge of the screen displays all the other apps that are running. Right click the one you want to close and select Close from the menu.
If you are actually using an app and have finished with it, move the mouse to the top edge of the screen in the middle. It then changes to a hand and you can click and drag down the screen. This shrinks the app to a thumbnail and when the mouse reaches the bottom of the screen, let go of the mouse button and the app disappears of the bottom as it closes.
Press the Ctrl+Alt+Del keys all at the same time and then select Task Manager from the menu that is displayed. Select the Processes tab and among all the running processes are the Start screen apps. Right click one and select End Task.
If you are running short of memory because you have a lot of programs running it makes sense to close one or more to free up memory. Start screen apps are different though and they are more like tablet apps and they are handled in a simlar way. You just run them switch from one to another and just leave them open in the background. This works like tablets and Windows manages the memory used. If necessary it can close an app to free up memory.
It's OK to leave Start screen apps in the background and it's much less of a problem than with traditional programs.