What is difference between Diff and Patch?

Diff creates Patch.

In simple terms, the diff command is used for comparing the differences between two versions of a file. The resulting file is called a patch, and typically is given (by the user) a “.patch” suffix.

This patch file then can be used on other copies of the “old” file by using the patch command, thus updating their “old” file(s) to match the “new” file(s).