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.