What's the difference between Normal & HD version of a game?

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http://androidforums.com – Just wondering if android apps works like iphone apps where the Normal version has the same quality on the phone as the HD version on the tablet. Or does it make a difference if I use the HD version of the app on the phone? Since we're on this topic, do I use the tablet version for galaxy note 2? (because of the large screen). :confused: (General)