Differences between iPhone OS v2 and v4?

Can any one say difference between the iPhone OS 2+ till 4 and what's the main difference in the OS as well as the d开发者_如何转开发evelopment in xcode..

Thanks

For the difference between the versions go to http://developer.apple.com/iphone/library/navigation/index.html#section=Resource%20Types&topic=Release%20Notes and see the Documents called iOS x.x API Diffs

In Release notes section of iOS Reference Library you can find "API diffs" docs for every iOS version starting from 2.1.

The answer of this question should be very much in detail but may be following my answer in other post will help you to understand how apple world is working:

Are there any Hardware Requirements to test iPhone Apps on your iPhone