Web Components have been around in some form or another for a couple years now but are now just getting closer to a finalized version. Frameworks like Angular, Ember, and Ionic (Built on Angular) all use custom elements/components in their frameworks to allow you to easily create content and controls.
This episode of Web.Dev takes you through an intro of the pieces that make up Web Components and using Polymer (a polyfill library) to create a couple examples of Web Components.
[02:24] Web Component - The four pieces
[04:26] Draft Status
[06:37] Getting setup for the demo
[10:56] Custom Elements
[16:56] Templates and HTML Imports
[26:41] Shadow DOM and Styles
[34:44] Next Steps