a
b
c
d
e

c
d

grep -F -x -v -f b.txt a.txt

http://unix.stackexchange.com/questions/28158/is-there-a-tool-to-get-the-lines-in-one-file-that-are-not-in-another

This works by using each line in b.txt as a pattern (-f b.txt) and treating it as a plain string to match (not a regular regex) (-F). You force the match to happen on the whole line (-x) and print out only the lines that don't match (-v). Therefore you are printing out the lines in a.txt that don't contain the same data as any line in b.txt.