ASP.NET Core is an open-source and cross-platform framework for building modern web applications using .NET. It is a redesign of the earlier Windows-only version of ASP.NET, and it supports various scenarios such as web apps, IoT apps, and mobile backends. It offers features such as high performance, middleware, minimal APIs, Blazor, and more. It runs on .NET, a free, cross-platform, and open-source application runtime.