For the last 15 years, whenever I wanted to copy one server to another (usually for backup), I've used mirror.pl, a program written in 1991 and not updated since. It's worked fine. I've also used gencopy, a program that I wrote for doing the same thing. But recently, mirror.pl was having a lot of difficulty with one particular copy, so I fired up google to look for a possible alternative. That led me to rsync.
I've heard of it, but I was never interested enough to find out what it was. I discovered that it does pretty much everything that mirror.pl does, plus I think it's faster, plus it works on the server that mirror.pl was having trouble with.
rsync is installed as standard on every Fedora install, and on raspbian you get it with apt-get install rsync.
I think I'll be using it a lot in future.