Using vimdiff to compare two files is fairly simple. Let's look at two files in animal_farm/ : animals/cat.py and animals/dog.py. We'd like to know what's different between the two.
Open the files with vimdiff:
$ vimdiff animals/cat.py animals/dog.py
You will be greeted to the following screen (how colorful the screen is will depend on your colorscheme):
You can see animals/cat.py open in ...