Coral UI:- It is a set of CSS and JS files designed and built for Adobe Cloud products. Coral UI provides a wide range of html components pre developed for your website like buttons, navigation bar, tool bar, tables, grid, dialogs etc. and save you time from developing these components.
Granite UI:- Granite UI is a foundation UI framework provided by adobe to develop modular, layerable and reusable components. Granite UI components are built with Coral UI. Granite is an open development project within Adobe which means any one from adobe can contribute to the code, discussions and issues.
http://www.aemcq5tutorials.com/tutorials/create-touch-ui-component-using-coral-ui-aem/
https://medium.com/@vsr061/develop-front-end-components-with-aem-coral-ui-2da905096cce
https://myaemlearnings.blogspot.com/2018/03/difference-bw-granite-ui-components.html?m=1