Price System:

Demand is the quantity of a commodity that consumers are willing and are able to buy at a given price at a given time period when all other things remain the same. A change in any of the other determinants of demand apart from the own price of the commodity (income of consumers, prices of related commodities, consumer tasted demand and shift the demand curve. A change in own price of the commodity will cause a change in quantity demanded and produce a movement along the same demand curve

Supply on the other hand is the quantity of a commodity that suppliers are willing and are able to offer for sale at a given price at a given time period when all other things remain the same. A change in any of the other supply factors apart from the market price of the commodity (prices of inputs, technology, taxes or subsidies, prices of other commodities, weather/natural phenomena and population of suppliers) will cause a change in supply and shift the supply curve. A change in market price of a commodity will cause a change in quantity supplied and produce a movement along the same supply curve.
 
At equilibrium, the market clears. That is, quantity demanded is equal to quantity supplied. A change in demand or supply will cause the equilibrium price and quantity to change.

Whenever the equilibrium price is realised to be unfairly high or low it may necessitate the setting of a price ceiling or a price ceiling or a price floor by government. Price ceiling is a legal price set below the equilibrium price.