rsync is a utility function provided with the Linux distributions I've used, and also available for Windows, but I had never encountered it in the MS-DOS/Windows operating systems I'd used. In essence, I see it as a very extended version of file copy or xcopy. It provides lots of options enabling conditional copying across a network1), with or without preservation of Linux permissions, with or without date changes, with or without preserving directories, and probably more. My first use of it was to make a quick backup of my desktop project files onto a thumb drive before leaving the office, but I've used it at home for syncing directories (or just plain transferring files) between machines in my residential group.
DESCRIPTION
Rsync is a fast and extraordinarily versatile file copying tool. It
can copy locally, to/from another host over any remote shell, or
to/from a remote rsync daemon. It offers a large number of options
that control every aspect of its behavior and permit very flexible
specification of the set of files to be copied. It is famous for its
delta-transfer algorithm, which reduces the amount of data sent over
the network by sending only the differences between the source files
and the existing files in the destination. Rsync is widely used for
backups and mirroring and as an improved copy command for everyday use.
And, important for trying things out,
As a special case, if a single source arg is specified without a desti‐
nation, the files are listed in an output format similar to "ls -l".
Getting started, one list of ten fifteen examples caught my interest.
Adding the capability to Windows:
Experimenting and learning by playing, I took some notes. One theme I published as a blog note, and will repeat here. Another, very basic theme on the importance of what one slashes I'll add once I've anonomysed it enough.