What is the relevance of elasticities of demand?

It is often easy to lose sight of the wood for the trees.  In micro economics we study how the interaction of producers and consumers contribute to the allocation of resources.  How much demand responds to changes in income, the price of substitutes and compliments as well as changes in the price of a particular product.  We must have a sound knowledge of all of these concepts.  However, we need to apply this theory to the reality of the market place to gain a deeper understanding.  Below are several sources to enhance your learning of applied economics.  (You only need to read the first few pages of the Wal-Mart paper to get the gist).

yED and strategies               PED              XED          Wal-Mart