Food is any substance ingested to give nutrition to an organism. In the dietary field, food is any material intended for consumption that contains nutrient content in sufficient quantity to supply the needs of the organisms. The basic functions of food are to increase the nourishment, to store energy, to promote growth, to digest and metabolize nutrients, and to improve appearance. The nutritional value of food substances depends upon their physiological properties, chemical nature and the proportion of certain elements present. Generally, it is considered that food is the most important of all the essential resources of life. Every living thing, be it a plant or an animal, can only live on food.

Animal foods like meat, fish, milk and eggs provide the basic nutrition required by an animal. Plants, like green and leafy vegetables, are also an important source of food, but their distribution and consumption are often prevented by humans because of the danger presented by insects and animals. Milk is one of the oldest sources of nourishment known to mankind. Its fat and protein components have many roles in the diet; animal sources are rich in proteins, while plant-based foods contain carbohydrates and fats, which are used for energy.

Food habits and the food supplies depend largely on the climatic conditions, culture and tradition, and on the availability of water and fertility of the land. A variety of diets are followed, including vegetarianism, which means avoiding or limiting animal products and fish and other animal-derived foods. Cereal is a staple food consumed by many people, especially in Asia and Africa; in countries with dry climates cereal is not eaten often except at special occasions. Rice is a grain eaten widely all over the world.