What it looked like was an email program with a list of subject names like mail folders, each containing subject lines of conversation threads. The threads were fully branched, replies under the correct messages, like Lemmy. Not a simple list, like email.
Also unlike email, the messages were posted publicly instead of to you.
There was a list of newsgroup names for different subjects, you’d pick which of those to get messages from to appear as the “mail folders”.
The names were in a hierarchy, so computer subjects were comp.something, hobbies/recreation were rec.something etc. a bit like website names, only back to front, general to more specific, e.g uk.rec.sheds, alt.startrek.fanfic , rec.humor, rec.humor.funny.
You’d download messages from (and upload your replies to) a server and it would share messages with other servers, like Lemmy federation. So each group would be a merge of all messages from all around the world. Effectively there would only be ONE alt.folklore.urban for instance.
Usually your isp would run a server and you’d use that.
At first it wasn’t mainly used as a way to share binary files encoded as text messages, but eventually that took over, isps dropped having servers and big paid ones took over.