Fomantic UI is a community fork and continuation of Semantic UI. It is mainly a JavaScript/CSS framework characterized by human readable (semantic) class names. Use this tag along with others, like [javascript] or [css] to narrow down the scope of your question.
Semantic UI is a front-end GUI framework that provides a collection of JavaScript and HTML/CSS classes to develop responsive UIs.
Semantic UI appears to not being actively developed. Fomantic UI is the community fork for maintenance and upgrades on top of Semantic UI.
Resources:
Links to the documentation for each element and style can be found in the sidebar of the "Getting Started" page.
As a fork of Semantic UI, the community-maintained Fomantic UI is equivalent to the parent repository at least until version 2.4. It may diverge starting from 2.5 and onward.