The comm utility reads file1 and file2, which should be sorted lexically, and produces three text columns as output: lines only in file1; lines only in file2; and lines in both files. It's sort of twist on diff. It's nice because there's no formatting to get in the way.
$ cat > 1.txt << __END__Thanks to Krishna Srinivasan for teaching me about a new old-school tool ;)
$ cat > 2.txt << __END__
$ comm 1.txt 2.txt