Add files to a commit
git add <FILES>
git mv or
git rm for moved or deleted files.
Show the modified files
Show the modifications in a file
git diff <FILE>
git commit -m "Description of the commit"
Fix last commit message
git commit --amend -m "Modified description"
Undo changes in a file
git checkout <FILE>
Get the list of commits
Reverse a commit
git revert <SHA>
This does not delete the commit, it creates a new commit that cancels previous changes. The history is preserved.
git reset <SHA>
This deletes the not yet pushed commits from the history, but the files are preserved. The commits are deleted, but the contents are still on the disk.
To also cancel the changes on the contents:
git reset --hard <SHA>