SharePoint supports two basic User Interfaces (UI) designs; The “Classic” experience, and the “Modern” experience.
The “Classic” experience is a highly customizable user interface that integrates SharePoint lists and libraries.
The “Modern” experience is mobile ready, (currently) limits the visual customization that can be done, makes the creators’ experience more intuitive, and makes the user’s experience more collaborative.
This means the “Modern” experience is ready to use, where the “Classic” experience requires a bit more configuration if defaults are accepted, and design and development if high customization is required.