Rdiff-backup. Diff, Patch, and Friends. Diff is designed to show you the differences between files, line by line.
It is fundamentally simple to use, but takes a little practice. Don't let the length of this article scare you; you can get some use out of diff by reading only the first page or two. The rest of the article is for those who aren't satisfied with very basic uses. While diff is often used by developers to show differences between different versions of a file of source code, it is useful for far more than source code.