Unison is a file-synchronization tool for OSX, Unix, and Windows. It allows two replicas of a collection of files and directories to be stored on different hosts (or different disks on the same host), modified separately, and then brought up to date by propagating the changes in each replica to the other.

If you just want to use Unison, you can probably find a pre-built binary for your architecture either on your favorite package manager or here:

http://www.cis.upenn.edu/~bcpierce/unison

If you want to play with the internals, have a look at the file src/ROADMAP.txt for some basic orientation.

Here's how to choose which version to use:

• The most recent public release (2.51, as of Summer 2019) is the default recommendation and should work well for most circumstances. Data-loss bugs, if any should come up (they have been rare, historically) will be fixed in this version, and it will be updated as necessary so that it always compiles with the latest OCaml release.
• Previous public releases are still stable and useable, but they will not be updated to new OCaml versions or have bugs fixed.
• The "master" branch of the github repo is generally extremely stable and safe to use, for those that don't mind compiling from source.