Unison is ‘based’ on rsync. This is important for me to understand as I previously assumed that Unison provided another sub-file differencing algorithm that makes rsync so fast and powerful. This is not the case.

One best summary of the reason why Unison exists comes from Blane Warrene at sitepoint.com: “Unison synchronizes rather than mirroring as rsync does—making it valuable for workstations as well as development and production servers. To boot—it works on OS X! [http://www.sitepoint.com/blogs/2005/03/28/replacing-rsync-with-unison/]”