Maxime Fabre wrote a comprehensive tutorial on Webpack on the madewithlove-blog.

... Is Webpack a build system or a module bundler? Well, it's both– and by this I don't mean that it does both I mean that it combines both. Webpack doesn't build your assets, and then separately bundle your modules, it considers your assets to be modules themselves.

http://blog.madewithlove.be/post/webpack-your-bags/