Lactose is a type of sugar found in milk and dairy products. It is composed of glucose and galactose, which are both monosaccharides (simple sugars). Lactose is broken down by lactase enzyme present in the small intestine, allowing individuals to digest it properly. Some people have a reduced ability to produce lactase as they grow older or due to genetic factors, leading to lactose intolerance - an inability to consume dairy products without experiencing gastrointestinal symptoms such as bloating and diarrhea.