Income Effect - In the short run, your income is fixed. If the price is reduced, you can afford more of the product without giving up other goods. A decline in the price of a product will increase the purchasing power of your 'money income' , enabling you to buy more of the product than before. A higher price has the opposite effect. Substitution Effect - Consumers tend to 'substitute' cheap product for dear products, margarine for butter, Levi's for Guess jeans.
Competition assumes many buyers.
The 'market' demand schedule is obtained by the summing of the quantities demanded by each consumer at the various possible prices. Factors other than price that influence the consumer decision of the amounts of a good or service demanded at a given price. A simple change in the level of price will result in a 'movement' along the demand schedule rather than a 'shift' in the demand schedule. Fewer consumers will decrease demand. Income - changes in 'money incomes' affects on demand is more complex.