(a) A change in own price of the commodity will lead to change in qunatity demanded or in other words, expansion or contraction demand. It leads to movement along the demand curve.
(b) Price of substitue goods and qunatity demanded of another subsitute has a positive relationship. Hence, an increase in price of the substitute good will lead to an increase in demand of the concerned good and vice-versa. It leads to a shift in the demand curve.