Nothing, really. It’s just a change of philosophy between Rails 1 and Rails 2. Before Rails 2, you had file.rhtml, file.rxml and file.rjs. In Rails, that changed to file.content_type.template_engine. So with file.html.erb, the content type is html and the template engine is ERb. rxml is now xml.builder and rjs should now (mostly) be js.rjs.