Other answers and a link explain what XIB files are, but not about specific changes.
The new .xib format in .xib format was introduced with Leopard (and Xcode 3.0) in 2007, but I believe that .nib remained the default, primarily for compatibility with older versions of Xcode and OS X. There was a transition from Xcode 3.1 on .xib as the preferred format. However, most of the documentation has not been updated, perhaps partly because the difference is quite small from the point of view of a regular developer.
I definitely reiterate that XIB files are much better than NIB files, especially when using version control. (Flat files are much easier to manage than catalog packages in most tools.)
Quinn Taylor Jul 01 '09 at 16:37 2009-07-01 16:37
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