Main site : https://zettelkasten.de/posts/overview/
Zettelkasten seems to be an information organizing method (or MindTrafficGeometry which is like wiki in the sense of being a dense hypertext of SmallPiecesLooselyJoined but unlike wiki in that these items are indexed by an abstract id number, partly based on a date, as opposed to ConcretePageNames.
So in that sense it's more like a blog. And depends on search rather than guessing the names of topics.
Forum : https://forum.zettelkasten.de/
Discussion on categories :
See also :