Mushroom

Mushrooms can grow in dark caves and shady areas. They are more readily available in swamp and mushroom biomes and the Nether due to its low light conditions. They have also been known to spawn in jungles. A brown mushroom alone will increase the air block it occupies by one light level. Red mushrooms are generally rarer than their brown counterparts.

Obtaining
Mushrooms can be collected by hand and planted on the ground in light levels below 13 and not directly underneath the sky. (except in the cases of mycelium or podzol blocks, where mushrooms will remain planted even under full sun).

Mushrooms grow only at light level 12 and below, except when planted on mycelium or podzol. Mushrooms grow at the same rate regardless of light level. Brown and red mushrooms grow at the same speed.

Planted mushrooms will grow into huge mushroom when bone meal is used on them provided they are planted on a dirt, grass, mycelium, or podzol block, and if they have sufficient room to grow. Mushrooms will pop out of the ground if water flows over them, they are pushed by a piston, or the light level goes above 12 and the block next to them updates.

The individual blocks of huge mushrooms will drop individual small mushrooms when broken by the player. The blocks cannot be set on fire and are unaffected by lava.

Mooshrooms are a unique mob that serve as a source for red mushrooms (when sheared) and mushroom stew (when "milked" by the player while holding a bowl).

Trivia

 * Sometimes you cannot place mushrooms. This is due to them requiring a light level dark enough to be grown on. However, you can grow mushrooms on Mycelium or Podzol at any point in the day.
 * Red mushrooms bear a strong resemblance to Amanita muscaria (Fly Agaric) mushrooms. Consuming Amanita muscaria will cause delirium and hallucinogenic effects if their psychoactive properties have not been reduced by parboiling. Similarly, red mushrooms in Minecraft caused damage to the player in the Survival Test.
 * Brown mushrooms are more common than red mushrooms in swamps, and this is balanced by mushroom biomes having mooshrooms, making red mushrooms more common there.
 * Mushrooms are one of the few blocks an Enderman can pick up.
 * Brown Mushrooms emit a very small amount of light but it's so small it has to be completely dark to notice it.
 * If The Player (in Creative mode) enters the Void at the top of the Nether, they may find mushrooms on top of bedrock blocks.
 * Often times mushrooms can be seen in a ring around trees, due to the shade surrounding it from the leaves.