>Is it a Wordpress replacement?
Textpattern is a content management system. It's not as fully-featured or as widely-deployed as Wordpress, but it can be used as a "write stuff and share files" CMS in the way that a lot of alternative content management systems can.
>If so, what differentiates it?
It's lightweight, agile, and works readily on most MySQL (or store brand derivative) + PHP servers. We like it. The community around it is excellent, too.
>Does it provide a GUI for non technical users?
Yes. Again, not as full-fat as Wordpress, and as with any CMS it takes a little while to learn the lingo, but it's a thoughtful interface with considerations given to workflow, efficiency and suchlike.
I've never used it. It bills itself as a "Content Management System", which I've always thought was an amorphous term. I suppose you could use it for a blog, wiki, or something similar.
Textpattern person here. It's readily usable as a blog, corporate site, etc. A wiki would be less straightforward, especially if you have multiple users doing stuff, since we don't have any revision history built into the core software.
Textpattern 4.0.0 (the first production version) was released in 2005, and we're currently working on Textpattern 4.8.8 for release in Q4 this year after PHP 8.1 lands at the end of November.
The 4.9 release series is also being worked on, we'll probably see the first cut from that branch in 2022.
* https://github.com/textpattern/textpattern (core software)
* https://forum.textpattern.com/ (community forum)
* https://docs.textpattern.com (docs)
* https://textpattern.co/demo (demo landing page)
* https://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=textpattern (CVEs on mitre.org)