GNU diff utilities:

   You can use the `diff' command to show differences between two
files, or each corresponding file in two directories.  `diff' outputs
differences between files line by line in any of several formats,
selectable by command line options.  This set of differences is often
called a "diff" or "patch".  For files that are identical, `diff'
normally produces no output; for binary (non-text) files, `diff'
normally reports only that they are different.

   You can use the `cmp' command to show the offsets and line numbers
where two files differ.  `cmp' can also show all the characters that
differ between the two files, side by side.

   You can use the `diff3' command to show differences among three
files.  When two people have made independent changes to a common
original, `diff3' can report the differences between the original and
the two changed versions, and can produce a merged file that contains
both persons' changes together with warnings about conflicts.

   You can use the `sdiff' command to merge two files interactively.