Demand Curve : 

A demand curve is a graph of the demand schedule. The demand curve is plotted from the information given in the demand schedule. The price is measured on the vertical axis and quantity per unit of time on the horizontal axis. The demand curve is a downward sloping curve because of the inverse relationship between price and quantity. All demand curves slope downward because of the law of demand: as price falls quantity demanded increases.

When the price of a good or service is the only factor that changes, the quantity demanded changes but the demand curve does not shift. When the price of a good decreases, everything else held constant, the quantity demanded of that good will increase. This change is just a movement from one point of the demand curve to another point on the same demand curve, not a shift of the demand curve. Change in the quantity demanded is a phrase that economists use to describe the changes in the quantities of a good or service that people are willing and able to purchase as the price of that good or service changes.

The demand curve shifts when income, tastes, prices of related goods, consumer expectations or the number of buyers changes. A rightward shift represents an increase in the quantity demanded (at all prices), whilst a leftward shift represents a decrease in the quantity demanded (at all prices). For instance, if the income of the consumer increases, then consumers buy more normal/luxury items and the demand curve shifts towards the right. Similarly, if a good becomes fashionable then the demand for the good will increase and the demand curve will shift to the right. While the demand curve is usually a downward sloping curve, sometimes the demand curve may be horizontal, implying perfectly elastic demand, or vertical implying perfectly inelastic demand.