Ideally, vapor rises from the stage below, where it mixes with the liquid from the stage above. The vapor from the stage below supplies the heat to vaporize the light components in the equilibrium mixture and the liquid from the stage above supplies the heat sink or cooling to condense the heavy components in the equilibrium mixture.
I'm sure there are textbooks and internet sites that explain it better than me, but that's how I think of it.
Good luck,
Latexman