Blazor is a framework for building interactive web applications using C# and .NET. It allows developers to write UI components using C# and HTML, and run them in the browser using WebAssembly or on the server using SignalR. It offers features such as component model, routing, forms, validation, dependency injection, and more. It is part of ASP.NET Core and runs on .NET, a free, cross-platform, and open-source application runtime.