Read carefully the rsync options, from
It explains it all in a lot of detail, so if you can't find something, check that you're not lazy on reading
yes, it can be confusing too, but please do read it first, and when you don't understand some option(s), post them and ask. If you're interested in taking incremental backups on the same media, without losing space for non-changed files, check out the --link-dest=DIR option. Hard links don't cost you lots of space like "real" copies do, but allow you to get, for example, a directory hierarchy by date and time that allows for viewing "complete backups by time" instead of just a bunch of files that are "the newest". Sort of what OS X's Time Machine does.
When your rsync stuff is ready, test it and pay close attention to the results, to make sure it really does not re-transfer unchanged files. For example without the -u (--update) option rsync would copy files again and again even if they didn't change, but with it (after the first copy, if the original file does not change) the original file is older than the newer (which is created at the copy process), and causes rsync not to copy it until it changes (and thus becomes newer).