Ice

"The slippery properties of ice make it a surprisingly useful block. Pour water across its surface and items and players can move super fast in the direction of the water’s flow. Try dashing across ice enclosed in a two-block-high tunnel, and you’ll zip along at twice the speed of a minecart! Why? Because of SCIENCE. Well, it’s either that or a bug relating to how ground friction is calculated, but I’m going with science - and only partly because I fear Jens’ kung fu training."

- Marsh Davies

Ice is a translucent solid block.

Natural generation
Ice can be found naturally as part of the landscape in snowy biomes from frozen lakes, rivers, and oceans. It can also be found in igloos, Ice Spikes, and Icebergs.

Snowy biomes
Water source blocks in a snowy biome eventually freeze into ice if exposed to the sky from directly above, the light level immediately adjacent to the water block on all sides is less than 13, and there is at least one horizontally adjacent non-water block. This can happen at any time of day, and in any weather condition. If the highest adjacent light level is 12, an ice block alternately melts and re-freezes when it receives a block tick.

Water also freezes into ice in mountains biomes above the level that snow forms.

Ice Bomb
When an Ice Bomb is thrown to the water, it transforms the water in a 3x3x3 cube centered around the projectile into ice. This works for source water or flowing water upon hit.

Obtaining
Ice can be easily destroyed without tools, but the use of a pickaxe speeds up the process. The block drops only when using a tool enchanted with Silk Touch. Frosted ice cannot be obtained (created from a Frost Walker enchantment). It can be broken instantly with Efficiency III on a Diamond Pickaxe.

Speed
Ice is slightly slippery, causing entities (excluding minecarts ) to slide, including items. This also allows for increased speed in water currents by placing the ice block under the water current. A player who runs and jumps repeatedly on ice travels faster than on any other block type.

When a non-full block is placed on top of ice, the block has the same "slipperiness" as the ice below it. This feature is intentionally programmed into the game. although, if ice is placed below soul sand, it just increases the slowing effect of the soul sand rather than making it slippery.

Creating water
Ice can be used to create water either by it melting or being broken. If there is another block directly underneath the ice block, the ice reverts to water when broken. Ice also melts into water if the light level immediately next to it on any side is higher than 11, from light sources other than sunlight (and regardless of whether there's a block below). $$, ice also melts when near a heat block, though heat blocks do not produce light. If ice melts or is broken in the Nether, no water is produced.

Trivia

 * When riding a pig on ice using a saddle, the pig moves incredibly fast, making journeys across long frozen lakes easier.
 * By sprinting and jumping while on ice and inside a 2-block tall tunnel, it is possible to move 16 blocks a second, twice as fast as a full-speed minecart. By replacing the 2-block ceiling with trapdoors, it's possible to travel 1000 blocks in 54 seconds, or 18.518 blocks per second. However, this drains the player's hunger bar extremely quickly at roughly 1 unit per second.
 * Boats move extremely fast on ice; however, it is difficult to slow down or change direction.
 * Sugar cane can be generated next to ice, though they drop as items if updated. This can be observed at random when running through a snowy biome.
 * Attempting to set ice on fire with a flint and steel causes no flames to appear, but the flint and steel's durability still decreases by 1. The same thing happens with glass and the sides of non-flammable blocks.
 * Ice is classified as a transparent block $$, and therefore does not conduct redstone.
 * Snow layers are the only transparent block that cannot be placed on ice.
 * When a player holds an ice block, the normals of the smaller model are flipped inside out, giving a strange effect.