A supertype is a characteristic of a card and any one of six words that may appear on a card's type line just before its card type[s]:
The supertypes all have rules text attached to them: More than four copies of a basic land can be included in a deck, and state-based effects dictate what happens if multiple Legendary permanents with the same name, or multiple World permanents of any name, are in play simultaneously. Some cards also refer to supertypes by prefixing them with "non", which means all cards that are not that supertype.
From the Comprehensive Rules
(Battle for Zendikar
(September 26, 2015
- 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.
- 205.4b An object’s supertype is independent of its card type and subtype, even though some supertypes are closely identified with specific card types. Changing an object’s card types or subtypes won’t change its supertypes. Changing an object’s supertypes won’t change its card types or subtypes. When an object gains or loses a supertype, it retains any other supertypes it had.
- Example: An ability reads, “All lands are 1/1 creatures that are still lands.” If any of the affected lands were legendary, they are still legendary.
- 205.4c Any land with the supertype “basic” is a basic land. Any land that doesn’t have this supertype is a nonbasic land, even if it has a basic land type.
- Cards printed in sets prior to the Eighth Edition core set didn’t use the word “basic” to indicate a basic land. Cards from those sets with the following names are basic lands and have received errata in the Oracle card reference accordingly: Forest, Island, Mountain, Plains, Swamp, Snow-Covered Forest, Snow-Covered Island, Snow-Covered Mountain, Snow-Covered Plains, and Snow-Covered Swamp.
- 205.4d Any permanent with the supertype “legendary” is subject to the state-based action for legendary permanents, also called the “legend rule” (see rule 704.5k).
- 205.4e Any permanent with the supertype “world” is subject to the state-based action for world permanents, also called the “world rule” (see rule 704.5m).
- 205.4f Any permanent with the supertype “snow” is a snow permanent. Any permanent that doesn’t have this supertype is a nonsnow permanent, regardless of its name.
- 205.4g Any scheme card with the supertype “ongoing” is exempt from the state-based action for schemes (see rule 704.5w).
- Wasteland has the text: ", Sacrifice Wasteland: Destroy target nonbasic land."
This means that wasteland can target any land that is not a basic land.