When you open Manage Nuget Packages, In online section there exists the "nuget.org" and "Microsoft.Net"
From what i understand is that "nuget.org" is a opensource packages that everyone can contribute and on "Microsoft and .NET" has only the Microsoft and .NET is only the packages that come directly from Microsoft. Is that right ?
nuget.org is the full list of available packages while "Microsoft and .NET" is a curated list of Microsoft-provided packages and Microsoft supported 3rd party libraries like jQuery.
See details here.