Minecraft wiki entry for Superflat, associated text: minecraft:mossy_cobblestone,250*minecraft:air,minecraft:obsidian,minecraft:snow — comma-separated list of block IDs.
minecraft:mossy_cobblestone — one layer of mossy cobblestone on layer -64.
250*minecraft:air — 250 layers of air, from layer -63 to layer 186.
minecraft:obsidian — one layer of obsidian, on layer 187.
minecraft:snow — one layer of snow, on layer 188.
minecraft:mountains — biome ID, in this case Mountains.
stronghold(count=7),village(size=0 distance=9),decoration,dungeon,mineshaft — a comma-separated list of structures.
stronghold(count=7) — this populates the world with seven strongholds at the default distance and spread.
village(size=0 distance=9) — because Mountains is an invalid biome for villages, this does nothing.
decoration — biome-specific decoration.
dungeon — allows dungeons to generate.
mineshaft — allows abandoned mineshafts to generate at the default placement frequency.