Difference between .NET Standard, .NET Core, and .Net Framework Class Libraries?

As you know when the .net core was launch from that time we have a .net core, net Standard new libraries are available in vs.

.net framework class library is old one which having .net framework 3.5, 4.0, 5,6 etc.

.net core class library is totally we using for .net core projects.

.net standard having all .net core libraries , for xamarin, mono we are use this.