Shows unstaged changes

git diff

Shows staged changes

git diff --staged

The same as the above command

git diff --cached

Shows the difference between the working directory and index (both staged and unstaged files)

git diff HEAD

Shows the difference between the commits

git diff a80982d 17d3352

Shows the difference between

login.frm

files from different commits

git diff a809822 17d3352 login.frm