diffutils

Diffutils includes four utilities: diff, cmp, diff3 and sdiff.

Diff compares two files and shows the differences, line by line.
The cmp command shows the offset and line numbers where two files differ, or cmp can show the characters that differ between the two files.
The diff3 command shows the differences between three files. Diff3 can be used when two people have made independent changes to a common original; diff3 can produce a merged file that contains both persons' changes and warnings about conflicts.
The sdiff command can be used to merge two files interactively.

Install diffutils if you need to compare text files.