# Describing the UI[](#undefined "Link for this heading")  
React is a JavaScript library for rendering user interfaces (UI). UI is built from small units like buttons, text, and images. React lets you combine them into reusable, nestable _components._ From web sites to phone apps, everything on the screen can be broken down into components. In this chapter, you'll learn to create, customize, and conditionally display React components.
### In this chapter  
*   [How to write your first React component](/learn/your-first-component)
*   [When and how to create multi-component files](/learn/importing-and-exporting-components)
*   [How to add markup to JavaScript with JSX](/learn/writing-markup-with-jsx)
*   [How to use curly braces with JSX to access JavaScript functionality from your components](/learn/javascript-in-jsx-with-curly-braces)
*   [How to configure components with props](/learn/passing-props-to-a-component)
*   [How to conditionally render components](/learn/conditional-rendering)
*   [How to render multiple components at a time](/learn/rendering-lists)
*   [How to avoid confusing bugs by keeping components pure](/learn/keeping-components-pure)
*   [Why understanding your UI as trees is useful](/learn/understanding-your-ui-as-a-tree)