Food

Food is used in Minecraft to restore health. Some types of food are used to make comestibles that you can consume (eggs, milk, sugar, wheat, and mushrooms are considered ingredients and cannot be eaten on their own). To enjoy a sumptuous repast in-game, select the desired food from the toolbar and right-click. Cake must first be placed as a block and can then be consumed by right-clicking.

While all food ingredients (with the exception of Milk) are stackable items, food items cannot be stacked in the inventory.

Notch recently suggested a food bar of some sort, which could mean that you will need to eat periodically, whether or not you've taken damage.

Ingredients
Eggs, milk, mushrooms, wheat, and sugar cannot heal unless they are crafted to make some of the food items above.