This document provides an overview of a UI 101 session which covers HTML, CSS, Sass, Bootstrap, and how to think like a UI developer. The session includes introductions to basic HTML syntax and structure, CSS syntax and properties like box model, positioning, flexbox, and media queries. It also compares CSS to Sass and provides examples of animating CSS and building a responsive website layout with common elements like navbar, sections, and footer. The document serves as a guide for the topics and code examples that will be covered during the UI 101 training session.