The Microsoft .NET Framework creates desktop apps and ASP.NET applications that run on the Internet Information Server (IIS). It was the first release of a controlled framework.

The .NET Framework is a programming language of computers firstly developed by Microsoft.

The Base Class Libraries are different for each managed framework implementation. Exception handling, strings, XML, I/O, networking, and collections are among the classes in the Base Class Library (BCL).

.NET Standard Definition:

.NET Standard is a formal specification of .NET APIs that are available on multiple .NET implementations. 

The BCL is implemented using the .NET Standard, which is a specification. Because this standard requires a .NET implementation, application developers will no longer have to worry about separate versions of the BCL for each managed framework implementation.

WPF, WCF, and ASP.NET are examples of Framework Class Libraries (FCL) that are not part of the BCL and thus not included in the .NET Standard.

The HTML standard and a browser have the same relationship as .NET Standard and a .NET implementation. The second step is to put the first into action.

As a result, in their managed frameworks, the .NET Framework, Xamarin, and.NET Core all use .NET Standard for the BCL. There will be new managed frameworks for .NET as the computing industry releases new hardware and operating systems. This standard ensures that application developers access a standardised set of APIs.

There is a version of the .NET Standard for each .NET version.

Porting programmes to various managed implementations and providing tooling is made easier by offering standard APIs.

Because all .NET implementations are required to support it, .NET Standard is defined as a single NuGet package. Because the tools use the same set of APIs for each iteration, tooling becomes easier. You can also create a single library project that multiple .NET implementations can use.

For platform-specific APIs, you can also write .NET Standard wrappers.