React is a popular front-end development tool that was originally created for Facebook, somewhere around 2010 (depending on the source you are looking at), and was open sourced in 2013 by Jordan Walke on JS ConfUS. From there on out React skyrocketed, and currently based on Stackdiary ranking it takes first place in front-end frameworks popularity.
From React documentation direct explanation what is React is as follows:
Simple React Component Example
At ExN we are working with React on a daily basis on many of our projects basically from the day we started. We have couple of React experts on board, who help our new team members to easily grasp even the most complex parts of React. We are constantly monitoring community for any new changes and tools due to our constant engagement with the technology, and we stay up to date with newest changes.