React is a UI Library in Javascript. Created by the folks at Facebook, it helps with creating webcomponents, but better than the webcomponents implemented in HTML5. React allows you to create composable view components, utilizing a unidirectional data flow, or one-way data flow. This differs to Angular that has a data binding components, which is two directional between controller and view.
Note: I call elements components, but now they're really called elements. Take that in consideration when I use the word component.