It's a tool for secure file transfer and synchronization -- securer than FTP which is just plain text transfer without any encryption. rsync is much faster than FTP too. It's syntax is very simple:
rsync [options] <source> <destination>
I suggestion you also read the man page for rsync. It's very clearly explained what it does and how it works. (in particular pay attention to how directories are treated and how to copy directories recursively. It's a bit tricky at first, but logical when you understand the concept clearly). The man page can explain better than I can.
From the terminal issue,
for more info.