What is React, and why is it used?

Direct answer

React is a JavaScript library for building component-based user interfaces that update declaratively when application data changes.

React lets developers describe a UI as a tree of components and update that UI by changing props or state instead of manually issuing DOM commands.

Component model: split the interface into reusable, composable pieces.
Declarative rendering: describe what the UI should look like for the current data.
Cross-platform model: React itself defines the component model; renderers such as React DOM target specific environments.