Vim on two (or three or four) files.
Each file gets its own window.
The differences between the files are highlighted.
This is a nice way to inspect changes and to move changes from one version
to another version of the same file.
See vim(1) for details about Vim itself.
When started as
gvimdiff the GUI will be started, if available.
In each window the diff option will be set, which causes the differences
to be highlighted.
The wrap and scrollbind options are set to make the text look good.
The foldmethod option is set to "diff", which puts ranges of lines without
changes in a fold. foldcolumn is set to two to make it easy to spot the
folds and open or close them.