Elasticity of demand refers to the inverse relationship between price and quantity demanded of a good. There are two main types of elasticity - elastic demand, where a small price change leads to a large quantity change, and inelastic demand, where a large price change results in a small quantity change. Elasticity can be measured using various methods including percentage, point, and arc methods to calculate price, income, substitution, and complementary elasticities. The elasticity of a good is determined by factors like availability of substitutes, level of expenditure, and time period. Elasticity analysis has practical applications in areas like taxation, monopoly pricing, and economic policymaking.