Click the above image to see an excellent visual explanation of how and why to use "whom" in a sentence. I love The Oatmeal's entertaining and quirky take on subjects as traditionally dry as grammar. Example:
The mastermind behind The Oatmeal is Matthew Inman. His self-written bio says it all:
"The Oatmeal's real name is Matthew and he lives in Seattle, Washington. He subsists on a steady diet of crickets and whiskey. He enjoys long walks on the beach, gravity, and breathing heavily through his mouth. His dislikes include scurvy, typhoons, and tapeworm medication."
Matthew acknowledges the help of a librarian for his "whom" comic:
If you like Matthew's grammar posters, you can buy The Oatmeal Grammar Pack.