The site is called Matricula Online. This site offers church registers (mostly containing birth, marriage and death records) from several “German or former German speaking regions in today’s European countries of Austria (with the most data), Germany, Serbia and Poland (around the former Breslau today Wrocław, Poland). It appears that Matricula has plans to expand, although I am not sure into which regions. To get an idea of their present coverage see the map (on the right) for more details on their current record offering locations.
To use the site simply go to Matricula Online and chose your prefered language option (English or German). The types and quality of their links will vary. All seem to be free.
The site offers hints and explanations of what you can expect. Note, the English version of the explanation is a bit cryptic (perhaps a Google Translate issue).