what's the difference beetween XIB and NIB file??????
Nothing (as of now). Files with the .xib extension are pronounced "nib" files.
Check out the wikipedia article here for a more in-depth explanation.
Check this out: http://speirs.org/blog/2007/12/5/what-are-xib-files.html
XIB is pronounced "nib".
A XIB file is XML. A NIB file is binary.
XIBs replaced NIBs.
There really isn't a difference between how they are used, but XIBs are now the standard while developing. I believe that XIBs are translated into NIBs when the application is compiled. They are both used the same way and are both edited in Interface Builder.
For more information about what a XIB is look here.
If you need more information you can find a ton of information about this subject if you use google.
XIB is actually XML file. NIB is binary. So there are problems when using SVN with NIB files. So XIB replaces them.