Swift version: 5.1
NIBs and XIBs are files that describe user interfaces, and are built using Interface Builder. In fact, the acronym "NIB" comes from "NeXTSTEP Interface Builder", and "XIB" from "Xcode Interface Builder". NIBs and XIBs are effectively the same thing: XIBs are newer and are used while you're developing, whereas NIBs are what get produced when you create a build.
In modern versions of iOS and macOS, NIBs and XIBs have effectively been replaced by storyboards, although you may still meet them if you work on older projects.
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Available from iOS 7.0
This is part of the Swift Knowledge Base, a free, searchable collection of solutions for common iOS questions.