Yogurt is basically a form of curdled milk much like sour cream and creme fraiche but with less fat.
Bacteria in the milk ferments and coagulates to thicken the milk to a creamy texture, adding a tangy, slightly astringent flavor. In commercial manufacturing, the friendly bacteria is added, but if you were to heat fresh milk and keep it at about 100 degrees F. for a few hours, it would naturally turn to yogurt.
Yogurt can be made from any variety of mammal milk, but is most often made from cow, buffalo or goat milk. Kefir is an alcoholic version of yogurt originally made from fermented camel milk, but now made from cow milk. Available in some natural food stores, kefir has an alcohol content of about 2.5%.
There are several types of yogurt with differing fat levels readily available in the market. Plain yogurt is generally made from cow milk and is unflavored and unsweetened. Flavored yogurt generally has fruit or flavoring added, along with plenty of sugar. The sugar is added not only for sweetness but to aid in preserving the fruit. Frozen yogurt is the yogurt version of soft ice cream. It is generally stabilized by the addition of gelatinwhen made at home.
All types of yogurt can be found in regular, low-fat and non-fat so you can choose your fat content.