A hook is a melodic passage that is repeated throughout a song and forms an earworm in the listener. A riff is a bit of wankery that interrupts the flow of a song and is a demonstration of the ego of the player.
I have no clue, but I heard an interview Terri Gross had done with Pat; he claims not to have written with a hook in mind. He stated they just came naturally.
Posting this here, and in Pat's thread, for those interested. Thanks for the tip it was out there.
EDIT: Man, I really like that interview and the insights into Pat's view of the creative and writing process, homages, new ideas, song structure, hooks, etc. Worth a quick listen. It isn't long.