Ice is a translucent, seemingly white block. It may extend a back wall below it as normal, but only up to ten blocks down when it forms naturally in water. This may depend on the degree of cold.
Ice will despawn kelp growing where it forms. Fish and sharks tend to avoid forming ice, but may become trapped anyway and will eventually despawn if not freed. If a blockhead is completely frozen in ice, it will be still, but will slide up to the surface if it tries to move. Blockheads can still perform normal mining and crafting tasks in ice.
A fractionally filled block of water will freeze into a full block of ice. Prior to version 1.4, these blocks would melt into a full block of water. In 1.4, it was changed so they instead melted into the partial amount of water it froze from. If mined and then placed however, it would again melt into a full block. If there isn't enough to freeze, the water will disappear instead.
Ice can be crafted into columns and stairs at a stonemason's bench.
While ice can be used for construction, its tendency to melt in warm air makes it impractical except under arctic conditions. It has no known use in crafting.
Ice can be used to transport water without using a bucket.
Because mining and placing ice will refill partial water blocks, ice is part of one method of generating unlimited water (and therefore ice).
As water within range of cold air will turn into ice, it's not possible to "ice fish" by digging down to liquid water. What happens instead is an expansion of the ice and allowing more of it to be dug out. It's theoretically possible to build an underwater structure or completely empty even a large body of water this way, although care should be taken to avoid being submerged by a thaw.