Blazor is a new web framework that allows web applications to be written in C# instead of JavaScript. It uses WebAssembly to run .NET code directly in the browser. Developers write Blazor apps using Razor components with HTML and C# code. At runtime, the Blazor runtime compiles the Razor components to WebAssembly, which generates the app's rendering tree and updates the DOM efficiently as the app runs. Blazor provides a way to build interactive client-side web UI using .NET instead of JavaScript.