Ubuntu Core is a minimal version of Ubuntu intended for large-scale deployments, bringing transactional updates through the "snappy" packaging system.
From the Ubuntu website:
Snappy Ubuntu Core is a new rendition of Ubuntu with transactional updates - a minimal server image with the same libraries as today’s Ubuntu, but applications are provided through a simpler mechanism. The snappy approach is faster, more reliable, and lets us provide stronger security guarantees for apps and users — that’s why we call them “snappy” applications.
Snappy apps and Ubuntu Core itself can be upgraded atomically and rolled back if needed — a bulletproof approach that is perfect for deployments where predictability and reliability are paramount. It’s called “transactional” or “image-based” systems management, and we’re delighted to make it available on every Ubuntu certified cloud.