Sort of. It predated the web, so calling it a "site" is wrong. Just like you can have an email application that's completely separate from your web browser, you can have a Usenet client that's also its own thing. Of course, people made web-based clients as time went on.

Your ISP ran a Usenet server that connected to other Usenet servers. The biggest problem with this system was that your ISP would automatically delete posts past a certain age. Following old threads was a pain.

Google Groups started as a Usenet archive where messages were kept forever. Google bought them and turned it into what it is now.