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spiderbatman: multitasking lets multiple apps run simultaneously. WP supports this for first party apps, but not for third-party ones, except in special cases. For example, I have an app (not in the store) that grabs a bunch of content from a website; I use it before I get on a plane so that I have some recent content to read. The app typically pulls about 120-150 web pages in, one at a time, but in a rapid-fire manner. If I start that running on my phone then press the Start key, my app is still loaded in memory, but it isn't grabbing any of those pages. If I press the Back key, the app picks up where it left off and keeps grabbing content. If WP had "true" multitasking, the downloads would continue while I was looking at the Start screen or using other apps.
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spaulagain: WP7 allowed you to resume an app by backing in to it from another app. Mango (WP7.5) added the ability to "fast app switch" by holding down the Back key for two seconds. However, both always killed and restarted the task when it was launched from the Start screen or the alphabetical app list. WP8 adds the ability for a app's developer to tell the OS that the app does not want to be killed and restarted when the icon is selected from the Start screen. In cases where the developer has done this, the app can either jump back to the page that was last active or ignore the back history and start fresh (as WP7.x would have done).
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1101x10: I know and I sometimes wonder why MS did it. I presume that it was to bring the same kind of power savings to Windows RT tablets that it did to WP and to make it easier for programmers to think in one consistent way for both the phone and tablets.