Food

Food are items that restore hunger points on consumption (as shown in the Hunger bar) and have a direct bearing on the player's health (see 'hunger' for more on effects). Except for milk, cake, and mushroom stew, food items and ingredients can be stacked in the inventory. Holding the "Use Item" button (default is Button 2, right mouse button) makes the player eat the food in their hand. With the exception of Golden Apples, food may not be eaten when the hunger bar is full. Note that cake must be placed like other blocks on a surface before being eaten.

Food
Eating food partly restores hunger (as shown in the bar) and to a hidden variable, saturation, which is an additional hunger bar, but one whose value will not exceed your current hunger level.

P.S. Mark is completely wrong!

Nourishment value
Nourishment is defined as the ratio of saturation to food points restored by a certain food item. To avoid wasting saturation points, more nourishing foods should be eaten when the hunger bar is more full. In particular, once the hunger bar starts rippling (zero saturation), "wasting" the visible hunger points is no longer an issue, because if you don't eat, they will be going away soon anyway.

Ingredients
The following items cannot be eaten on their own. Instead, they are crafted with other ingredients to produce certain food items.

While the following ingredients can be crafted with others, they can be consumed to induce or nullify certain effects on the player.

History
Before Beta 1.8, food did not have an eating animation, simply disappearing from the player's hand as if a block had been placed. Food also restored a set amount of hearts instantly - a player could return from the brink of death during a fight almost immediately by consuming a few porkchops or bowls of stew. To keep a certain level of challenge to the gameplay, Notch prevented players from taking advantage of this instant recovery by ensuring food (with the exception of the ingredients) did not stack in the inventory.

When the Hunger Bar was added in Beta 1.8, food became an even more vital resource as the player was required to eat regularly to avoid dying of starvation and regenerate health after taking damage. With the exception of Cake, Milk and Mushroom Stew, food could now stack to 64 in the player's inventory. Both types of mushroom could now be farmed by using bone meal on either individual mushroom to produce a Huge Mushroom that dropped more fungi when harvested.

The first Beta 1.9 pre-release revealed two new sources for mushrooms - an entire biome filled with Huge Mushrooms and the Mooshroom, a strange red cow with red mushrooms growing on its back that could be milked with a bowl to collect Mushroom Stew and shorn to harvest the red mushrooms. However Mooshrooms can only be found in Mushroom Biomes which are extremely rare and difficult to find.

In the second pre-release of Beta 1.9, Jeb said that Milk now acted as a "clear-everything drink" to nullify the effects of potions and poisons. Spiders also dropped eyes that could be crafted with sugar and a brown mushrooms to produce brewing ingredient, fermented spider eyes.

In the 1.3 Version, Cocoa Beans, that were once only obtainable from treasure chests in Dungeons and Abandoned Mineshafts were made into a crop that can be farmed, thus making all food items a renewable resource.

From Minecraft 1.4 (Snapshot 12w34a), potatoes and carrots can be obtainable from zombies (rare drop) or harvested. Potatoes can be cooked to make baked potatoes, and harvesting potatoes may give 0-2 poisonous potatoes. Also, carrots can be crafted into golden carrots. And also in 1.4, Pumpkin Pies are added to the game. These are not cooked, but rather crafted by combining a pumpkin, an egg and sugar. From this, pumpkins are now a food ingredient.

Trivia

 * You can continue breaking blocks even while eating, the actions simply occur simultaneously.
 * Each and every kind of food is fully renewable.
 * If eating while climbing vines/ladders or swimming directly up or down, you will not be slowed.
 * You can eat while you are sleeping.
 * The eating animation appears only in the first-person view.
 * When you eat stew, the brown particles of the bowl spray out as well, making it seem like you are eating the bowl. Also, it makes a "chewing" sound effect rather than "slurping" (as milk and potions do) further suggesting you are actually eating the bowl. Despite all this, the bowl is retrieved with no damage whatsoever.
 * The player cannot consume any food/drink in Creative mode, except milk. They are only able to drink potions.