Snow

Snow is a block 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 regenerating during snowfall. Snow melts in the same pattern as ice.

If the snow is on a grass block, the grass will turn white. This does not affect grass growth. 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.

Snow will not form on top of glass, ice or piston.

After the 1.8 update, snow biomes were not in game, as they were in development. Notch said that snow was rather ugly and he was going to make it more awesome. In 1.0.0 snow biomes had come back.

Bugs
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. Snow can also spawn on top of oceans in rare occasions and will disappear when hit by a boat. This snow will spawn on the top layer of water not one block above it, giving it the appearance of clumps of snow floating in the water.

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 1/5 chance of having worldwide snowfall. Since Beta 1.5 however, Snow can occur in arctic Biomes, and since 1.0 (?) tundra biomes and 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 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.
 * 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.
 * The "topSnow" block's thickness is based on its data value.
 * As of Beta 1.9 pre5, the top of Grass' and Snowy Grass' textures were slightly changed.
 * A Snow Golem can be used to generate an unlimited amount of snow without the need to explore more land or wait for snowfall.
 * Floating snow, when hit, will cause all other floating snow tiles to disappear.