High Level Features
Authentication as a Service
Centralized login logic and workflow at a single & well-secured place.
Single Sign-on / Sign-out
Single sign-on (and out) over multiple application types like web or mobile.
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 social identity providers like Google, Facebook etc, as well as integration for enterprise identity management systems via SAML and WS-Federation.
The most important part - every aspect of IdentityServer can be customized to fit your needs. Since IdentityServer is a framework, you can write code to adapt the system in a way it makes sense for your scenarios.