Snow was a mechanic from Ice Age and Coldsnap. Cards would have the snow supertype, like Snow-Covered Mountain, and in turn there were cards that cared/rewared you specifically for how many Snow cards you had, like Skred.
June 25, 2015 12:51 p.m.
TheRedMage says... #3
Two main reasons permanents have this supertype:
- So that other cards can reference or count them (for example, Skred, Ronom Hulk, etc.)
- Mana produced by sources with the snow supertype (such as Boreal Druid, Arctic Flats or Snow-Covered Mountains) can be used to pay for "Snow Mana", on abilities like the ones on Gelid Shackles or Scrying Sheets
The supertype doesn't do anything by itself though. It's just there so it can be referenced.
June 25, 2015 12:52 p.m. Edited.
Coinman1863 says... #4
Second example that you might see is Scrying Sheets. There is even a special symbol for snow mana, as seen on the mentioned card.
June 25, 2015 12:53 p.m.
TheRedMage says... #6
Rules that are relevant to the supertype snow (and we're going pretty deep here):
107.4. The mana symbols are 
, 
, 
, 
, 
, and 
; the numerals 
, 
, 
, 
, 
, and so on; the hybrid symbols 
, 
, 
, 
, 
, 
, 
, 
, 
, and 
; the monocolored hybrid symbols 
, 
, 
, 
, and 
; the Phyrexian mana symbols 
, 
, 
, 
, and 
; and the snow symbol 
.
107.4h The snow mana symbol represents one generic mana in a cost. This generic mana can be paid with one mana of any type produced by a snow permanent (see rule 205.4f). Effects that reduce the amount of generic mana you pay dont affect S costs. (There is no such thing as snow mana; snow is not a type of mana.)
205.4a A card can also have one or more supertypes. These are printed directly before its card types. The supertypes are basic, legendary, ongoing, snow, and world.
205.4f Any permanent with the supertype snow is a snow permanent. Any permanent that doesnt have this supertype is a nonsnow permanent, regardless of its name.
These are all the rules that reference snow. As you can see, there is nothing that makes these cards have a different mechanical behavior thant they would if they did not have the subtype. The subtype is just there so that other cards can reference it.
June 25, 2015 1:11 p.m.
And the last thing that comes to mind and hasn't been mentioned: You can put as many Snow-Covered Mountains in a deck as you want, just like you could with regular mountains (and of course, all other basic land types).
TheRedMage says... #1
Snow is a supertype with no rules baggage. Basically if something is a "Snow Creature" or a "Snow land" it will behave the same way as if it didn't have this supertype.
June 25, 2015 12:48 p.m.