One area of Microeconomics that we refer to throughout the course is the concept of elasticity.  It is important to understand the overall concept of elasticity being how responsive the quantity, either demanded or supplied, is to a change in something else; this can be the price of the good or one of the non-rice factors affecting the demand or supply.  If you can understand the basic concept and remember the standard definition then you can work out the definition of the 4 types of elasticity you need to know about (price elasticity of demand, income elasticity of demand, cross-price elasticity of demand and price elasticity of supply).  Below is the presentation we used in class while studying elasticity.