Differences between files, with your favourite editor
Some people complain about cmp, diff and other tools because the output is not user friendly. Ok, if you have the same thoughts and you like VI(M), this is a lucky day for you:
$ vimdiff file1 file2
Starts vim in diff mode, highlighting the differences between the two files. Both files will be showed at the same time by means of a vertical split (:vsp).
A good way to copy changes from one file to the other. In order to do that, it would be useful a good knowledge of copying, cutting, pasting, folding, unfolding and other features of vim