Bucket

Buckets are tools used to carry water, lava, or milk.

Usage
Empty buckets can be used to "pick up" a water or lava source block by doing on a block of the relevant type. Similarly one can do when looking at a mooshroom or a cow to fill a bucket with milk.

A bucket filled with a source block can then be used to place its source block contents in the empty block next to the side of the block the player is currently looking at, or replacing the block looked at for some replaceable blocks.

A bucket full of milk can only be emptied by drinking it.

An empty bucket can be used to empty a full cauldron, filling the bucket with water.

Trivia

 * In the Pocket Edition, buckets can hold any block, with the data value corresponding with what will be placed. Since the Bucket of Air doesn't place anything, this slot is used for the milk bucket. Keep in mind all of the special buckets can only be obtained with an external editor. They have the normal bucket sprite.