Leaves

Leaves are a type of block. They are abundant in normally generated maps, appearing on every tree, and they are a Renewable resource. Leaves have multiple textures, varying slightly in color; however, each individual leaf block will use the same texture on all six sides.

Leaves are available for use in Creative and Beta mode. They are often used in making plants, foliage, and tree houses. Leaf blocks are very weak, can be readily destroyed by hitting them with any tool, are extremely vulnerable to explosions and are highly flammable. As of Beta 1.2, leaves in snowy areas have a different texture. It is possible, but extremely unlikely for a leaf block to stay on fire after catching on fire.

Destroying leaves yields saplings 6.25% of the time which can be planted by the player in order to grow new trees.

In Beta as of 1.7 leaves can be collected using Shears. Using shears collects the leaves with a single click (unless you are in air and/or underwater, which will make them almost instant to harvest using shears), making it faster than the sword. Note that placed leaves still disappear after some time like normal leaves if there is no wood near them, though for the next release player-placed leaves will persist Jeb confirmed through Twitter. Leaf blocks can be up to four blocks away from wood and not disappear.

Leaves can be destroyed faster than with your hands or any other tool aside from shears by using a sword, though swords lose double normal durability when used on leaves.

Since the color of leaves can vary depending of biomes, some trees may have more than one leaves color on the same tree.

Alpha
In Alpha, leaves would decay using an algorithm that checks whether it has a solid or leaf block underneath it or is adjacent to a solid block. If it doesn't, it must be connected to a leaf block that is also supported as long as it is no further than five blocks away from said support.

Beta
In Beta, leaves decay according to a simple formula: leaf blocks decay unless they are within 4 blocks of a log block, and connected to the log by other leaf blocks. Thus, harvesting the wood from a tree will cause the leaf blocks to vanish randomly one by one. Before Beta 1.7 leaves could not be harvested without mods.

Trivia

 * In "Fancy" graphics mode, leaves are marked as transparent, but in "Fast" graphics they are opaque. You can see the effect by trying to look through a tree while toggling the graphics mode.
 * Before 1.6, the above made it possible to place torches and ladders on leaves in "Fast" graphics and switch back to "Fancy" graphics without making them come off.
 * Similarly, in "Fast" graphics, getting stuck inside leaves (usually by using bonemeal) will suffocate you, while on "Fancy" graphics leaves will not suffocate you. For the full list of things leaves do differently depending on graphics mode, see Transparency.
 * Leaves were used as windows before Glass was implemented.
 * In Beta, while hitting leaves, clouds and Snow cannot be seen behind the block you are hitting.
 * Occasionally, leaves will grow through other blocks, acting as though they are still connected to the tree. They may also completely replace blocks they try to grow into.
 * Leaves do not prevent a sapling from growing. As long as the five blocks above a sapling are either leaves or air, it will grow provided it has enough light.
 * Hostile Mobs can spawn on leaves on "Fast" graphics.
 * In SMP, leaves always behave as if running in "Fast" graphics mode. This means torches can be placed on leaves and hostile mobs have a chance to spawn on them, even when running "Fancy" graphics.
 * Leaves can be supported by a different tree if the original has decayed, even if that means the new tree is of a different species (e.g. a birch tree can support redwood leaves after the original redwood has been cut down).
 * In the default textures, the leaf to air ratio is about 3:2 for normal/birch leaves, and about 9:5 for pine leaves.
 * Before 1.7, leaves could only be obtained via inventory hacking, or the /give command on multiplayer. This is no longer so.
 * Leaves do not prevent Chests from being opened. Even when in "Fast" graphics.