Sri Sri Ravi Shankar: When someone is in love with somebody, they keep repeating their beloved's name, this is called jaap. And when they sing their beloved's name, it is called sankirtan.
Everyone hums sometime or the other, whether in the shower or while doing some chores - that is sankirtan, when the song arises from the heart.
When someone's name keeps coming up in the heart, it is called jaap. It is good to do jaap, whether the mind is engaged or not; it has a flavor of its own.