Cheese is basically curdled milk. To "curdle" a liquid means to make it congeal. When a liquid is curdled it typically separates into lumps known as "curds" with the remaining liquid portion called "whey". Curds are largely composed of the protein called casein.
They also contain other proteins, lactose, fat and calcium.

Cheese is made by adding a coagulating agent to warm milk, such as an acid, like lemon juice, or the enzyme rennet. The enzyme rennet is naturally found in the stomachs of nursing mammals and it helps it the digest its mother's milk. Cheese has been around for many thousands of years. Its origin stretches way back into prehistoric times. One theory for its discovery is that a traveler stored milk in a container made from a young goat's stomach. The container therefore would have naturally contained rennet. Such bags are still used today by shepherds to carry wine. The traveler carried it around for a while and the air was quite warm. When he got thirsty and tried to take a sip, nothing came out. He then cut open the bag and found cheese inside which the rennet had curdled.

There are many different types of cheese, which can be separated into many different categories.

Main Categories of cheese.

Hard
Semi-hard
Soft
Semi-soft
Fresh
Aged or Ripened

The difference in hardness is controlled by water content. If the water is pressed out or allowed to evaporate by aging, it gets harder.

Another criterion is the source of the milk. Cheese is made from the milk of cows, sheep, goats, buffalo, yaks, mares, etc.

Some cheeses are ripened with the aid of fungi or bacteria. They often have a strong odor.

Cheese makes up a significant part of the diet of many countries in the world.

Most of the largest consumers of cheese per person are located in northern Europe.

Germany, Finland, France, Holland, Norway and Great Britain are prominent examples. Large consumers from other areas include Italy, Greece, Turkey, Israel, Argentina and Australia. Interestingly enough, countries in the Far East, like China, Japan and Thailand do not include much cheese in their diet. It is thought that may be because people from those areas tend to be lactose intolerant and can't properly digest the cheese.