a riff is the main bit to a song....think the opening bit to "Heartbreaker" by Zep or a riff can be the main bit in "Iron Man"...you know the part right after Ozzy says "I am Iron Man" or the very start of "Gimme Three Steps" by Skynyrd. Iron Maiden takes licks and makes riffs out of them. :rolleyes: Think of "The Trooper" but it is still a riff.
a lick is little , generally short lead parts....think of when zep comes out of the wild gong and congo middle part of Whole Lotta Love and it goes "dunt! dunt!-- Lick" "dunt! dunt!-- Lick" "dunt! dunt!-- Lick" "dunt! dunt!-- Lick" "dunt! dunt!-- Lick" then the main riff. (i lost track of how many dunts and licks but I think you get the picture)