ReactJS is a JavaScript library developed by Facebook. It is used for building reusable UI components. React JS official documentation defined it as follows:

React is a library for building composable user interfaces, it supports and encourages the creation of reusable UI components, which presents data that changes over time by using declarative syntax.

React is used by most developers as the "V" part of the MVC architecture. React abstracts away the DOM from you, offering a simpler programming model and off course better experience.

One of the selling points of React is that it can also render on the server using Node. It can power native apps using its framework called React Native. It implements one-way reactive data flow, which reduces the boilerplate making its comprehension easier than traditional data binding.

It uses a concept called Virtual Dom, that selectively renders subtrees of nodes upon state changes. It does the least amount of DOM manipulation possible in order to keep your components up to date.