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.