Casync is

1. A combination of the rsync algorithm and content-addressable storage 2. An efficient way to store and retrieve multiple related versions of large file systems or directory trees 3. An efficient way to deliver and update OS and container images over the Internet 4. An efficient backup system

casync uses well-known concepts implemented in a variety of other projects (primarily git and rsync), and puts them together in a moderately new, nice way.