React is a UI library developed at Facebook to facilitate the creation of interactive, stateful & reusable UI components. It is used at Facebook in production, and Instagram.com is written entirely in React.
One of it's unique selling points is that not only does it perform on the client side, but it can also be rendered server side, and they can work together inter-operably.
It also uses a concept called the Virtual DOM that selectively renders subtrees of nodes based upon state changes. It does the least amount of DOM manipulation possible in order to keep your components up to date.
In this article on My Blog, we’ll look at how to set up a basic application with React and also explore what makes React thick.