Cheese is a dairy item made with milk, brought to a desired consistency through any of several processes that coagulate the milk protein casein, typically with the addition of the enzyme rennet. Other methods of producing cheese involve curdling milk by adding products such as lemon juice, vinegar, bacteria, and sometimes fungus.
Cheese comes in a number of varieties, with differences between them including the age of the cheese, it's texture, the addition of ingredients (such as spices, fruit, vegetables, wine, etc.), the animal that produced the milk (usually cows, but sometimes goats and sheep), and other differences.
It is unknown how cheese was first discovered or produced. However, there is an enduring theory that the first man to have made cheese had done so accidentally. According to this theory, cheese was discovered in a time and place where food items were stored in animal skins. When one man wanted to store milk, he used a stomach instead of a skin. After a little time, the milk interacted with the rennet in the stomach, causing the milk to curdle, and the resulting food item was found to be palatable.