React is a JavaScript library for building user interfaces.
Just the UI: Mostly used for the V in MVC. Makes no assumptions about the rest of your technology stack, it is easy to try it out on a small feature in an existing project
Virtual DOM: Uses a virtual DOM diff implementation for ultra-high performance. Can also render on the server using Node.js - requires no heavy browser DOM.
Data Flow: Implements one-way reactive data flow which reduces boilerplate and is easier to reason about than traditional data binding