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Demand is the quantity of a good / service are willing to buy at a given price in a given time period 

LAW OF DEMAND - There is an inverse relationship between price and quantity demanded, as price increases, Qd decreases and vice versa assuming Ceteris Paribus 

Reasons for downward slope

   - income effect - inability of income to stretch, less able to buy same quantity of goods/services as before when demand was lower. 
 - Substitution effect - as price goes up, consumers switch demand to other subsitutes 

Price is the only factor which causes expansion/contraction on demand curve

non-price factors which shift demand include

• population
• advertising 
• substitute prices
• income
• fashion/tastes
• Interest rates
• Complement's price