Demand refers to a consumer's willingness and desire to purchase a good or service based on its price. There are different types of demand, including cross, joint, composite, and income demand. The law of demand states that demand is inversely related to price - as price increases, consumer demand decreases, and vice versa. A demand schedule numerically lists the relationship between price and quantity demanded, while a demand curve graphs this relationship, with price on the y-axis and quantity demanded on the x-axis.