You may discard a Forest card rather than pay Snag's mana cost. Prevent all combat damage that would be dealt by unblocked creatures this turn.
Official Rulings for Snag
8/1/2008 : You may discard any card with the land type Forest, including Snow-covered Forest or non-Basic lands like Tropical Island, Murmuring Bosk, or Stomping Ground. You can even discard a card that is a land in addition to other types, such as Dryad Arbor.