Wastes is not a land type. If something asks you to name a land type, you can't choose Wastes.

Because Snow-Covered Wastes is a basic land, you can include as many of them as you like in your Constructed decks.

In Limited events (including Sealed Deck and Booster Draft), Snow-Covered Wastes must be in your card pool to be included in your deck. You can't add Snow-Covered Wastes to your card pool in the same way that you can add other basic lands.

Snow is a supertype, not a card type. It has no rules meaning or function by itself, but spells and abilities may refer to it.