Comparing files and list out the different value in output

Dear all.
Sorry to interrupt, but I do have no idea about this
I am new developer in linux system
so I would like you guys could give me a hand

Thanks

ok the question is
Example
a.txt > cat, dog, human, apple


b.txt > cat apple


comparing with a.txt and b.txt ………dog and human is missing on b.txt .. the output result list out : dog , humen … since b.txt is missing as comparing with a.txt

How can i develop ?


Thank you very much

man diff