The document discusses reusable Ember components and widgets. It begins with an introduction to web components and how Ember components have evolved from before and after Ember RC6. It then provides steps to build a reusable progress bar component in Ember, including defining a template, passing parameters, customizing the component, adding content and actions. Key aspects covered are using a 'components/' prefixed template name, parameterizing components, extending the Ember.Component class for customization, using {{yield}} to add content, and the sendAction method to invoke actions on the hosting controller.