Snow

Snow is a block modifier that is found in biomes that have snowfall, namely Tundras and Taigas. 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, but if the snow is on a grass block, the grass will turn white. 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.

Snow regenerates during snowfall.

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.

Snow will not form on top of glass or ice.

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, similar to destroying a block of floating sand. Also in beta 1.6.5 some blocks can not be placed on snow.

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. Since 1.5 however, Snow can occur in arctic Biomes.
 * Due to a missing feature in the Halloween Update, snow did not regenerate in versions Alpha 1.2 to Beta 1.4_01, but returned in Beta 1.5, as part of the new weather features.
 * 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.
 * Snow melts near light sources (except sunlight), up to 3 blocks from lava and glowstone, 2 blocks from torches, and it won't regrow 2 additional blocks away.