Usenet used to be ubiquitous in the 90's, and still well into the 2000's. It was the all-encompassing forum system before any other practical online forums even existed. While quite limited (you were pretty much limited to ascii text posts, with any binary attachments such as images being frowned upon), it was a very handy and useful way to have online conversations with people about a topic.
And the best part of it was that the forums were very centralized. With this I don't mean that their physical location on a server was centralized (the exact opposite, in fact; all usenet news group content was distributed, shared and synchronized among thousands of usenet servers from all around the world). What I mean is that if you wanted to find discussion about a certain topic (like a programming language, or a hobby), there was usually one major newsgroup for it, so all conversations about that subject were centralized on one (or at most a few) newsgroups that anybody could follow and participate in.
And the best part was that you didn't need to create accounts. Just connect to the usenet, search for the newsgroup you wanted, and start reading and posting. No extra hassle.
Those days are pretty much gone. ISP after ISP have been terminating their Usenet servers during the past decade or so. For example here in Finland, ten years ago basically all ISP's offered a Usenet service. Nowadays almost none of them do, and even those that do, have an extremely limited subset of all newsgroups (eg. limited to only Finnish newsgroups, and often even just a small subset of them.)
Perhaps nowadays the only widely available method for accessing Usenet is Google Groups. This is not a very good implementation because it lacks hierarchical threading, making it hard to follow conversations. On the other hand, it's pretty much the only option left.
Nowadays Usenet has pretty much been replaced with WWW-based online forums. There are advantages and disadvantages to this.
The major advantage is, naturally, that a WWW forum has no limitations on the type of content that it can support (other than the limits of WWW itself.) This is a major step-up from Usenet, and it's an excellent thing. There are of course other advantages like for example it being easier for the forum to have moderators (and for the forum software to offer tools for those moderators.)
The major disadvantages, on the other hand, are that there are thousands and thousands of random forums out there, and nothing is centralized, and you need to always create a separate account on each one of them. If you have a quick question about a certain topic, you can't simply search for a forum on said topic and make a quick post and get an answer. No, you always have to jump through the hoops of creating an account and have it validated... And this quite often for one single post you wanted to make, with a good likelihood that after you get the answer you will never have the need to post there again. This is a real hassle.