I hate to ruin it for everybody, even the book readers, but....
-- There are three dragons. One for each Targaryen to ride. Of course, we know Daenerys is a Targaryen. The other two Targaryens are Jon Snow and Tyrion Lannister. We know that only fire can kill the White Walkers, so of course they will ride the dragons and kill the walker invasion. And the final battle over who will take the throne will be between Daenerys and Jon. Hence, "A Song of FIRE and ICE".
Obsidian kills the White Walkers as well. Also known as DragonGlass and large deposits of it can be found on DragonStone which is the old home of the Targaryens.
Everyone knows it's coming down to Daenerys and Jon Snow.
