Webpack is a module bundler created by Tobias Koppers to help create big and modular frontend JavaScript applications.
Its main difference from other solutions is its support for any type of module system (AMD and CommonJS), languages (CoffeeScript, TypeScript, and JSX) and even assets (images and templates) through loaders.
You read it right, even images; if in a React application, everything is a component, in a webpack project, everything is a module.
It builds a dependency graph of all your assets, serving them in a development environment and optimizing them for production.
JavaScript is a language based on the ECMA Script specification that, until version 6, still didn't have a standard definition of a module. This lack of formal standards led to a number of competing community standards (AMD and CommonJS) and implementations (RequireJS and browserify).
Now, there is a standard to follow, but unfortunately there is no support for it in modern browsers...