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 not hinder natural grass regrowth. Snow will not damage tilled and hydrated field areas, nor will it protect those plowed fields from being "trampled." Gravity-affected blocks (sand and gravel) will not fall if the block below them has snow on it.

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

Destroying snow with a shovel yields 1 snowball, which can then be crafted into snow blocks or thrown. If it is destroyed with something other than a shovel nothing is dropped.

If snow forms on top of a cactus, the cactus will still cause damage when stepped on or touched. Cacti cannot grow if there is snow on top of them.

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