A component is something which encapsulates certain snippets of handlebars templates that are to be used again and again. In such case, a JavaScript is not necessary to be written. Just define the entire handlebars template and use the component that is created. Components make it easy to reuse the code, create widgets, tags in or not in the W3C, and much more. Components in ember.js are basically versions of web components.
Defining a component in Ember.js: Ember generate component component_name;