There are many types of cheese and just as many methods for making it. Although some cheeses undergo more steps and require more time to make, cheese in its most basic form involves curdling milk and then separating the solids from the whey. What happens after this point determines the variety of cheese as well as the taste, texture, color, and aroma.
Curdling milk is the first step to separating the solids (fat and some protein), from the liquid (whey protein and water). There are two ways to curdle milk, either with acids, like lemon juice or vinegar, or with an enzyme such as rennet.
Using an acid to curdle milk will yield small, crumbly curdles like you will find in ricotta cheese, queso fresco, or Indian paneer. Simply adding an acidic component to milk will not cause curdling to occur, but in the presence of heat curds will quickly form. Once the curds form, the liquids are drained away and the solids are processed or pressed into a mold.
Rennet may be the oldest method of curdling milk as it is found in the stomach linings of ruminants, which were used to store and transport milk in ancient times. Rennet is an enzyme that denatures the proteins in milk, causing curds to form. Curds that are formed with rennet have a gel-like consistency, allowing them to be stretched and molded unlike curds formed with acid. Cheeses with good melting qualities, such as mozzarella, are produced with rennet. Today rennet is mass produced by genetically modifying bacteria to produce the enzyme. This allows for an unlimited amount of rennet to be produced for a much lower cost.
Once the curds are separated from the whey, they must undergo processing to produce the distinct varieties of cheese that we are familiar with. Fresh cheeses (such as ricotta and paneer) are simply drained or pressed and may have a touch of salt added, but generally do not undergo any further processing.
Many cheeses are heated and salted to further drive out more moisture. The more moisture that is driven out of the curd, the "harder" the cheese becomes. This also allows the cheese to last longer without spoiling and the salt can provide extra flavor.
Cheeses such as mozzarella undergo a stretching process, with aligns the proteins and develops protein "fibers," which give the cheese a stringy texture.
Milder cheeses, such as colby or gouda, go through several washings to reduce the acidity and allow the mild, creamy flavor to shine through.
Cheese curds are often pressed into molds to create cheese wheels, blocks, or other shapes. At this point, cheese can be eaten fresh, brined for storage (as with varieties like feta), or ripened to further develop the flavor.
The ripening or aging process is responsible for much of cheese's varietal flavor differences. Cheese is stored under controlled environmental conditions allowing natural microbes to transform the proteins and other comments in cheese into new and highly flavored compounds. Many cheeses are inoculated with specific strains of bacteria or mold to produce a desired flavor, color, or even texture. Gasses produced by bacteria in swiss cheese become trapped within the cheese and produce the classic holed texture. The distinctive blue veining in blue cheese is also caused by bacteria that have been inoculated into the cheese.
Between the type of microbe used, how it is applied, and how long it is left to age, there are endless possibilities for cheese flavors and textures. It is because of this that cheese making has become such an art form throughout history.