Snow is a thin cover that can be placed on any solid block.
When there is air or any other passable node directly below it, it falls. When there is a Dirt with Grass under it, it will turn it into Dirt with Snow. Snow next to fire extinguishes it eventually. Snow is destroyed when any liquid flows into its space.
Snow is created on top of pine needles when a pine sapling grows on Dirt with Snow. Or Dirt with Snow, Snow or a Snow Block was next to the pine sapling when it grew. Because pine trees are renewable, snow is thus renewable, too.
As a crafting ingredient