Comparing files and updating them via PATCH

diff
To compare 2 text files or 2 directories

A patch file contains the deltas (changes) required to update an older version of a file to the new one. The patch files are produced by running:
$ diff -Nur originalfile newfile > patchfile
or
$ diff -N -u -r originalfile newfile > patchfile

To apply a patch:
$ patch -p1 < patchfile for an entire directory tree
$ patch originalfile patchfile for a single file

cmp
To compare 2 binary files

$ diff3 FIRST-FILE REFERENCE-FILE SECOND-FILE
To compare 2 files with the referenced one