Different between sdk and runtime in .net core?

• A .NET runtime, which provides a type system, assembly loading, a garbage collector, native interop and other basic services.
• A set of framework libraries, which provide primitive data types, app composition types and fundamental utilities.
• A set of SDK tools and language compilers that enable the base developer experience, available in the .NET Core SDK.
• The 'dotnet' app host, which is used to launch .NET Core apps. It selects the runtime and hosts the runtime, provides an assembly loading policy and launches the app. The same host is also used to launch SDK tools in much the same way.

The SDK is all of the stuff that is needed/makes developing a .NET Core application easier, such as the CLI and a compiler.

The runtime is the "virtual machine" that hosts/runs the application and abstracts all the interaction with the base operating system.

Only the latter is required to run the application, but the former is needed to develop the application.

A .NET runtime is a software component that implements the execution model of a programming language, which provides a type system, assembly loading, a garbage collector, native interop and other basic services. This is the "virtual machine" that hosts/runs the application and abstracts all the interaction with the base operating system.

The SDK includes everything we need to build and run .NET Core applications, such as the CLI and a compiler, using command-line tools and any editor (like Visual Studio). The runtime is also included in the SDK

In short, the runtime will allow your OS to run compiled your programs, and the sdk will allow you to compile programs.

When we say runtime we basically mean the CLR.

SDK is the combination of CLR + debug/code libraries. 

Inshort if you just want to host your app on a windows server then you just need Runtime not SDK.

If you want to code and run your app lets say in your local system then you need SDK.