Just files and folders.
Create small web pages, blogs and wikis.
Not much to learn.
Installing Yellow is just unzipping one file. Create blogs and wikis in a browser, write about the things you like. Content is simply text files, there's no database. The API for developers allows you to write plugins and more. Sometimes the best things are free. Yellow is free and open source, available on GitHub.
Yellow is made of files and folders. A flat-file CMS. Learn more about it.