The XML syntax inside of JavaScript is called JSX; check out the [JSX syntax](/react/docs/jsx-in-depth.html) to learn more about it. In order to translate it to vanilla JavaScript we use `<script type="text/babel">` and include Babel to actually perform the transformation in the browser.
The file `build/helloworld.js` is autogenerated whenever you make a change. Read the [Babel CLI documentation](http://babeljs.io/docs/usage/cli/) for more advanced usage.
If you want to use React with [browserify](http://browserify.org/), [webpack](https://webpack.github.io/), or another CommonJS-compatible module system, just use the [`react` npm package](https://www.npmjs.com/package/react). In addition, the `jsx` build tool can be integrated into most packaging systems (not just CommonJS) quite easily.
We also have a wiki where the community contributes with [workflows, UI-components, routing, data management etc.](https://github.com/facebook/react/wiki/Complementary-Tools)