Leaves

"Leaves were one of Minecraft's earliest additions, arriving alongside logs, ores and sand way back in 2009 in version 0.0.14a_01. They've changed a lot over time, first being tweaked to drop saplings, then to decay when deprived of their trunk, then to drop apples, then even to have water drip through them when it's raining!"

- Duncan Geere

Leaves are blocks which grow as part of trees.

Obtaining
Leaves can only be obtained with shears or Silk Touch enchanted tools.

Natural generation
Leaves occur naturally on trees throughout the Overworld. Oak leaves also generate in woodland mansions.

Usage
Leaves from trees spontaneously decay (disappear) when they receive a block tick, if they are not connected to any wood block, either directly or via other leaf block, with a maximum distance of 4 blocks. Player-placed leaf blocks will never decay.

Leaves which decay or are destroyed without using shears usually drop nothing, but yield saplings 5% (1/20) of the time. Jungle leaves drop saplings 2.5% (1/40) of the time. Oak and dark oak leaves also have a 0.5% (1/200) chance of dropping an apple. Rates are increased by the Fortune enchantment. Leaves that are burned do not yield saplings or apples.

Leaves take on a different shade of green depending on the biome in which they are placed.

Leaves are always transparent to light but cannot be seen through when the graphics mode is set to "Fast"; the transparent regions are instead black. They diffuse sky light, causing the shadows they cast under trees.

ID
Leaves use two different block IDs. They use  (block 18) for oak, spruce, birch and jungle; and they use   (block 161) for dark oak and acacia. Leaves use block data or block states to determine their type and state.

Block data
Block 18

Block 161

Block states
Block 18 (minecraft:leaves)

Block 161 (minecraft:leaves2)

Original leaf decay algorithm
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.

Trivia

 * 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 chests from being opened.
 * Some trees seem to occasionally drop apples or saplings without being destroyed due to creation of leaf blocks not supported by wood.
 * The frost texture in Minecraft: Bedrock Edition is not an instant transition. Instead, the leaves slowly shift from their original color to the frosty one.
 * Leaves disappear when pushed by pistons.
 * Leaves Z-fight with blocks such as carpet that are placed on top; a bug which Mojang decided not to fix.