If you use Angular, you annotate your classes with @Component. If you use React, you extend React.Component. If you use Polymer, it's because you want "to make the most of Web Components, a powerful new platform feature for extending HTML and componentizing your apps."
If you ask me, it sounds like Web Components are kind of a big deal when it comes to modern web development.
In this talk, Scott Davis (Principal Engineer with ThoughtWorks) discusses the shift from page-centric architecture (think jQuery) to component-centric architecture (think encapsulated, reusable, domain-specific custom HTML elements). You'll learn about the four core W3C APIs baked into (nearly) every modern browser -- Custom Elements, HTML Imports, Templates, and Shadow DOM. You'll also see how several modern web frameworks apply their syntactic sugar atop these core platform technologies.