Deco is a page composition system based on semantic HTML and a grid system. Instead of using custom XML namespaces and a templating language, Deco uses plain (strategic) HTML.
Taking end user experience simplification as a final goal, Deco approach to editing was introduced back in 2008. Since then this new rendering paradigm has actively been developed.
Deco brings a completely new approach to page editing in Plone. Page layouts are composed of tiles: small boxes of text, images, lists, forms, polls, and … anything. Tiles refers to the dynamic parts of a page: this is something that can be rendered individually or merged into a page. For those cases where you currently create a new content type - you would create a new tile instead.
Deco allows a full-page editing interface, it replaces Kupu, lets you manipulate DOMs, uses TinyMCE inside text files and eliminates the need for grid management. Deco framework also brings drag-and-drop UI which allows content editors to add different elements to their pages without having to do complex things in the page editor.