Target land becomes a 3/3 Elemental creature with flying until end of turn. It's still a land.
1/24/2013 If the target land hasn’t been under its controller’s control continuously since the beginning of his or her most recent turn, that land won’t be able to attack and its abilities won’t be able to be activated. In most cases, that means it will no longer be able to be tapped for mana that turn.
1/24/2013 Hydroform doesn’t affect the land’s name or any other types, subtypes, or supertypes (such as basic or legendary) the land may have. The land will also keep any abilities it had.
1/24/2013 Effects that modify power and/or toughness but don’t set them to a specific value (like the one created by Giant Growth), power/toughness changes from counters, and effects that switch a creature’s power and toughness will continue to apply. This may happen if the land was already a creature when Hydroform resolved.