Demand refers to how much quantity of a good or service is desired by consumers.

Supply is the amount of a good or service producers are willing to offer at a particular price.

There are several determinants which influence the quantity demanded and supplied of goods and services. However, our focus is first placed on price because of its pivotal role in markets. Assuming all these other determinants are held constant, ceteris paribus,  the quantity demanded, is the amount of a good or service consumers are willing/can afford to purchase at a given price. And the quantity supplied, is the amount of a good or service producers will provide at a given price.