Taken from IdentityServer4 Official Docs
IdentityServer4 is an OpenID Connect and OAuth 2.0 framework for ASP.NET Core.
It enables the following features in your applications:
Authentication as a Service
Centralized login logic and workflow for all of your applications (web, native, mobile, services).
Single Sign-on / Sign-out
Single sign-on (and out) over multiple application types.
Access Control for APIs
Issue access tokens for APIs for various types of clients, e.g. server to server, web applications, SPAs and native/mobile apps.
Support for external identity providers like Azure Active Directory, Google, Facebook etc. This shields your applications from the details of how to connect to these external providers.
Focus on Customization
The most important part - many aspect of IdentityServer can be customized to fit your needs. Since IdentityServer is a framework and not a boxed product or a SaaS, you can write code to adapt the system the way it makes sense for your scenarios.
IdentityServer is officially certified by the OpenID Foundation and part of the .NET Foundation.
Installation or Setup
Detailed instructions on getting identityserver4 set up or installed.
Step by step guide
This is a good step by step guide for setting up asp net Identity and Identity server for authorization and authentication.
You can also follow the official docs examples and IdentityServer4 quickstart samples on Github