Semantic UI is described as 'UI framework offering over 50 UI elements and 3000+ CSS variables. Integrates with React, Angular, and more, providing responsive design and easy theming' and is a CSS Framework in the development category. There are more than 50 alternatives to Semantic UI for a variety of platforms, including Web-based, Self-Hosted, Mac, Windows and Linux apps. The best Semantic UI alternative is Bootstrap, which is both free and Open Source. Other great apps like Semantic UI are Tailwind CSS, Bulma, Material UI and Foundation.
Both are front end frameworks, but Foundation is more popular and widely used.