Snow

Snow is a block modifier. When a map is generated, there is a chance that snow will be falling (dubbed "Winter Mode" by players). Any block which doesn't have another block vertically above it at any height will develop a layer of snow. Snow can be removed in the same way that any other block is destroyed. Placing a torch will melt any snow around it and will prevent it from developing. This is also the case for lava, active furnaces, and fire. It melts in the same pattern as ice.

A layer of snow will hinder natural grass regrowth. Snow will not damage tilled and hydrated field areas, nor will it protect those plowed fields from being "trampled." It is also unaffected by snow blocks, and will fall through and leave snow piled on the area below the snow.

This was added on July 9th, 2010, along with the addition of ice.

Destroying snow with a shovel yields 1 snowball, otherwise nothing is dropped.

Trivia
Before the Minecraft Alpha 1.0.5 update any blocks placed on snow would appear one block above the snow.