twtxt is a decentralised, minimalist microblogging service for hackers.
That is what is written on the official documentation. When I created my microblog I briefly read the spec and decided to implement a very similar format and expand a little bit on it. I will refrain from using the word "improve" as improvements are subjective.
What I like about the premise of twtxt is that, in it's core, it's extremely simple and that is a very welcome change. Because of this, even the support of Markdown that I've added to picoblog is questionable (I think the addition of the image tag even more so).
There are services like twt.social that took the idea of twtxt and adjust/transform the slightly hacky nature of twtxt into something more "appealing" (subjective) to the masses with a proper UI, file uploads, federation and so on. There are also services unrelated to twtxt but that remind me of it like subreply that embrace the simplicity but keep the UI and the less hacky nature.