When you use Process.Start, it starts a new Windows application - which means that the operating system looks at the file you told it to deal with, finds the appropriate application to open it and starts that. in this case, it's a URL, so Windows will automatically open a browser window - exactly as if you had typed "www.youtube.com" into a windows explorer address bar. You cannot prevent that, except by changing the default internet browser for the whole of windows.
If you want to "open" a website without displaying it, you will have to either use a WebBrowser control in a non-console app, or the
WebClient Class (System.Net)[
^] and process it manually.