Snow

Snow is a block modifier added in version 1.0.4 on July 9, 2010, along with ice. It is found in biomes that have snowfall, namely Tundras and Taigas. Any block without another block vertically above it at any height will develop a layer of snow. Snow can be removed like any other block. Placing a light source will cause snow around it to melt and will prevent them from developing. Snow 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 on. Gravity-affected blocks like sand and gravel will not fall if the block below them has snow on it.

Destroying snow with a shovel yields one snowball, which can then be crafted into snow blocks or be thrown. If it is destroyed with anything 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 it.

Due to a missing feature since the release of biomes (in the Halloween Update), snow does not regenerate any longer.

Glitches
Floating snow can be found above lakes in snowy biomes. This is presumably caused by the generation of underground lakes near the surface. The lake removes the block which the snow rested upon, but not the snow. The snow can be walked through and fallen through, occasionally causing the player to unexpectedly fall into a lake from what appeared to be a solid ledge. The floating snow creates an interesting effect and can be used to create realistic looking tables, and other furniture. If you put a block on floating snow, all of the nearby floating snow will disappear.

Trivia

 * Prior to the Halloween Update, snow could only be found on maps that were in Winter Mode. When creating a new save, a map had a chance of having worldwide snowfall.
 * Before the Minecraft Alpha 1.0.5 update any blocks placed on snow would appear one block above the snow.
 * In earlier versions of Minecraft, a player could easily harvest snow by placing a block of water. By doing so, the player received a large amount of Snowballs with little effort.
 * Notch mentioned that he might make snow tiles break when fallen hard upon.
 * Redstone (Dust) cannot be placed on snow without breaking the snow.