It's very easy to run an MD5 sum on the file on each side of a synchronization path to see if they differed. Rsync isn't just making sure that one or more files are the same on either end it's actually using a complex algorithm to compare parts of each file and, thus, significantly reduces the amount of time and bandwidth necessary to synchronize them. However, there's much more going on under the hood than many people realize.
Give it a source and target directory, along with a method of communication, and it will make the directories identical.
