2019 April 18, Microsoft Blazor Project leader Daniel Roth announces that Blazor is in official Preview status, it is no longer experiment.
With this newest Blazor release we're pleased to announce that Blazor is now in official preview! Blazor is no longer experimental and we are committing to ship it as a supported web UI framework including support for running client-side in the browser on WebAssembly.
(from Daniel Roth's blog)
The original blazor.net website is gone but the URL is redirected to the new Microsoft Blazor web page here
What is Blazor?
Server-side Blazor and Client-side Blazor
In the newer Blazor version, the old Server Component (or called as ASP.NET Core Razor Component) is named back to Server-side Blazor. The old client Blazor is named to Client-side Blazor.
Server-side Blazor runs on the server via SignalR
Client-side Blazor runs client-side on WebAssembly
Please read more information from Daniel Roth's blog.