Files with the .NUPKG extension contain source code packages that can be used to create development project components in a Microsoft Visual Studio environment that uses the NuGet extension. NUPKG files are based on NUSPEC files, which consist of sets of DLL libraries.

The NuGet Package Manager extension allows you to use and manage third party libraries in MS Visual Studio in .NET projects. The most popular libraries added in this way to projects are Microsoft Entity Framework, jQuery, or JSON. The NuGet extension can be added to the development environment through the Extension Management window in the Tools menu.