An Ice Bubble is an enemy found in both Twilight Princess and Phantom Hourglass. They are basically a regular Bubble that has been frozen. Because of this, they will freeze Link upon contact. While Link is frozen, his hearts will slowly deteriorate. They can be defeated with a Sword slash or an Arrow from Link's Bow.
In Phantom Hourglass Ice Bubbles appear exclusively within the Temple of Ice. If they come into contact with Link, they will freeze him on contact. They can be defeated with projectiles or exclusives, including the Bow and Arrow or Bombs. Alternatively, if Link hits an Ice Bubble with the boomerang, they will lose their cloud of ice briefly, bouncing around on the ground floor. At this point they become vulnerable to Link's sword attack.