Falsetto is a light, airy register above modal voice. Head voice is fuller, more resonant.
Falsetto: 'hooo' like an owl — light, no pressure. Head voice: 'maaa' with resonance in the head.
Distinction gives the singer control over expression. Falsetto = vulnerability. Head voice = power.
Falsetto: Bee Gees, Prince in 'Kiss.' Head voice: gospel singers, Jeff Buckley in 'Hallelujah.'