This is the brief explanation of Snappy on the main Ubuntu site: (source)
Ubuntu Core is a new rendition of Ubuntu for the cloud with transactional updates. Ubuntu Core is 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 to systems management that is perfect for container deployments. It’s called “transactional” or “image-based” systems management, and we’re delighted to make it available on every Ubuntu certified cloud.