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You might find this website interesting - it talks about the iPhone 6's screen resolution in a straightforward manner with lots of illustrations.
You probably will not notice anything different other than the larger size. It’s the same 16:9 ratio as older iPhones except for the oddball 4S with its 4:3 ratio display. This makes it easy to scale up. Also, there is auto-scaling built into iOS 8, so it should be a non-issue for the most part, unless perhaps you are using an iOS 6 app. If the new devices had markedly lower pixel density that the older ones, it might be an issue. However, the iPhone 6 Plus has a 401 ppi display, so it won’t be.
Keep in mind that Android devs deal with many, many variations of screen size and resolution, and it is almost always something that is dealt with well enough that it is a non-issue for Android users. Apple just added two variations - I think iOS devs should be able to handle that easily.