Documented in Volume 1 of the UNIX Programmers Manual.
% comm file1 file2
Use this on sorted files to produce a list of those lines present in file1
which are not common to file2. The first column in the list is lines found
only in file1. The second column is lines found only in file2. The third
column is last and shows lines found in both files.
% comm -23 file1 file2
displays only column 1 (lines in the file1 not in file2).
% comm -12 file1 file2
display only column 3 (lines common to both files).