Action extensions in OS X Yosemite: Explained

[iMore.com]

Action extensions promise to rework your Yosemite workflow by putting your content front and center instead of your apps​

OS X's Yosemite's new Markup tool is an example of a new class of technology called Action extensions. Action extensions promise to save you time and make you more productive with the software you're already using.

Full story from the iMore Blog...