I recently came across an interesting article talking about the @2x image in iOS. So to summarize that, if your app supports both retina and non-retina displays then you should include on image with a regular size for the non-retina display (ex. backImage.png) and another one with higher resolution for the retina display (ex. backImage@2x.png).
NS stands for NextStep, Which is the company created by Steve Jobs after he was fired from Apple in 1985. After Jobs came back to Apple, he brought the technology he created at NextStep with him which became later a naming convention for all the classes in Objective-C. For example, NSString, NSURL, and NSArray.