Web Components are a collection of standards which are working their way through the W3C:
HTML Templates, a native templating system which allows reusable HTML code using the new
Custom Elements, which allows authors to define and use custom HTML elements
Shadow DOM, allows authors to create an independent new DOM tree nested inside another DOM tree
HTML Imports, which allows authors to embed HTML documents in other HTML files, using the
At present, full support is only available in Chrome and Opera. However, given the potential benefits Web Components bring to the table, it might be a good idea to start experimenting with the APIs now, and help get constructive feedback and tests back to the spec writers. Check Can I Use or Webcomponents.org for up to date browser support status.