git diff (no parameters)
Print out differences between your working directory and the index.
git diff --cached:
Print out differences between the index and HEAD (current commit).
git diff HEAD:
Print out differences between your working directory and the HEAD.
git diff --name-only
Show only names of changed files.
git diff --name-status
Show only names and status of changed files.
git diff --color-words
Word by word diff instead of line by line.