Fast way of finding lines in one file that are not in another?

The comm command (short for “common”) may be useful comm - compare two sorted files line by line

#find lines only in file1
comm -23 file1 file2 

#find lines only in file2
comm -13 file1 file2 

#find lines common to both files
comm -12 file1 file2 

The man file is actually quite readable for this.