Concept of Demand:

As human beings we satisfy our wants by purchasing and consuming goods and services.  In ordinary terms, demand simply means a desire, a wish, or a want.  In economics, demand goes beyond the expression of mere desire, wish or want.   It is the desire, wish or want, backed by the ability to pay for what you desire, wish or want i.e. effective demand.

Definition of Demand

Demand is the quantity of goods or services that consumers are willing and are able to buy at a given price within a specified period of time when all other demand factors remain unchanged.  Hence demand could simply be a set of prices for a good or service with a corresponding set of quantities.