[changes between working tree and staging index]
git diff
[changes between staging index and last commit (HEAD)]
git diff --cached
[changes between working tree and last commit (HEAD)]
git diff HEAD
[view commit history]
git log
[compare changes of two commits]
git diff <commit-sha-first>..<commit-sha-second>
[example: git diff 37eb860f14f853843d4f75fe8ea55544c64b2f28..990d4041d98e51f3a9fe02625aee65977fe2db9f]