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Peek refers to the way in which you can 'peek' at your Hub and notifications without leaving whatever you were already doing.
For example, in the picture above, swiping up from the centre-middle of the screen to the middle-right hand side of the screen activates Peek and shows notifications waiting in the Hub. The platform also uses a Peek-like dissolve animation as you swipe from the bottom to the top of the device to unlock it.
Flow, on the other hand, is BlackBerry's name for the way in which people navigate the BlackBerry 10 OS, flowing from task to task, peeking at things along the way.