What is the difference between git am and git apply?

Both the input and output are different:

• git apply takes a patch (e.g. the output of git diff) and applies it to the working directory (or index, if --index or --cached is used).
• git am takes a mailbox of commits formatted as an email messages (e.g. the output of git format-patch) and applies them to the current branch.

git am uses git apply behind the scenes, but does more work before (reading a Maildir or mbox, and parsing email messages) and after (creating commits).